Differ with vs. Differ from

One differs with a person in the matter of opinion. 

I beg to differ with your interpretation. (matter of opinion)

The President also differed with members of Congress over this issue. (matter of opinion)

A person or thing differs from another in certain respects; it is unlike another in certain respects.

Delhi differs from Chennai in many ways. (in certain respects)

Laws on pollution differ widely from one country to another.

During vs. While

During and while both mean through a period of time.

Use during before a noun that names an event or a period of time. 

They work during the night and sleep by day.

You're not allowed to talk during the exam.

Use while before a clause.

I gained a lot of weight while I was on holiday.

I read it while you were drying your hair.

Rates for vs. Rates of

When rates means prices charged, it should be followed by for instead of of

State officials report that soaring rates for liability insurance are forcing cutbacks in the operations of everything from local governments and school districts to day-care centers and recreational facilities. - correct

State officials report that soaring rates of liability insurance are forcing cutbacks in the operations of everything from local governments and school districts to day-care centers and recreational facilities. - incorrect


When rate is used as a standard of measure, it should be followed by of instead of for

High rates of sick leave are a concern for all types of organizations. - correct

High rates for sick leave are a concern for all types of organizations. - incorrect

Because vs. In that

You use in that to introduce an explanation of a statement you have just made. 

e.g. I'm lucky in that I've got four sisters

When we use because, we are focusing on the reason. Because is used to express a simple cause effect relationship 

e.g. The boy was absent because he was ill.

Teratomas are unusual forms of cancer in that they are composed of tissues such as tooth and bone, tissues not normally found in the organ in which the tumor appears. - correct

Teratomas are unusual forms of cancer because they are composed of tissues such as tooth and bone, tissues not normally found in the organ in which the tumor appears. - incorrect

Where vs. In which

Where is used to refer to places only; to refer to events use in which

Stars like the sun can continue to shine steadily for billions of years because their light and heat are produced by nuclear fusion, in which  titanic releases of energy result in the loss of only tiny amounts of mass. - correct

Stars like the sun can continue to shine steadily for billions of years because nuclear fusion produces their light and heat, where titanic releases of energy result in the loss of only tiny amounts of mass. - incorrect

Idioglossia is a phenomenon, incompletely understood at best, where two persons develop a unique and private language with highly original vocabulary and syntax. - incorrect

Idioglossia is a phenomenon, incompletely understood at best, in which two persons develop a unique and private language with highly original vocabulary and syntax. - correct

For vs. Because

Punctuate for and because differently.

The coordinating conjunction for means because. 
Put a comma in front of for but not in front of because.

My room is cold for the heater is broken. - incorrect
My room is cold, for the heater is broken. - correct

My room is cold because the heater is broken

Few Quick Tips

1. Do not use since with an amount of time. Use for.

• I have been here since six weeks - incorrect
• I have been here for six weeks - correct

2. Do not use the simple present in a sentence containing since. Use the present perfect.

• I am here since January 5. - incorrect
• I have been here since January 5. - correct

Look at vs. Look to

Look at implies to turn the eyes in a certain direction so as to see; to use one's sight.

• He turned and looked at her.
• I’d like a skin specialist to look at that rash of yours.

Look to (someone or something) implies To rely on, turn to or refer to them or it, to expect

• Children look to their parents for help.
• This looks to be a very difficult assignment.

Maybe vs. Probably

Maybe

adverb

written as one word, defined as possibly or perhaps.

used to indicate uncertainty or possibility.

• Maybe it won't rain.
• Maybe no one will come to the party.
• Maybe you were right after all.

Probably
adverb

used to mean that something is very likely

• Sam was probably right.
• I'll probably be home by midnight.
• The infection was probably caused by swimming in contaminated water.

Economic vs. Economical

Economic is not same as economical.

                                                          

Economical means frugal, careful in management, not exaggerated or excessive; it has to do with saving.

• the most economical method

    

Economic means of or relating to production, distribution or management of wealth, pertaining to financial matters of a country or household. It means connected with the science of economics: relating to trade, commerce, the production and distribution of wealth, etc.

• economic problems
• a period of economic expansion.

Known to vs. Known as

Known to: when we want to talk about some fact that people have learned.

• Lions have been known to occasionally kill large beasts like elephants, giraffes, and rhinos.
• Elephants are known to communicate with seismics, vibrations produced by impacts on the earth's surface or acoustical waves that travel through it.

Known as: when we want to talk about the name of somebody or something or its definition.

• Some male Asian elephants are tuskless and are known as Muknas
• Lions are big cats known as the "King of the Jungle". 

Especially vs. Specially

Especially means 'particularly, to a degree beyond others'. To stress on exceptional quality, use especial or especially.

• He did especially well in national games. (A noteworthy performance)

Specially means 'for this purpose, or to this end, and no other'. To stress on the distinctive purpose of something, use special or specially.

• This laboratory is built specially for research. (A distinctive purpose)

Subject Verb Agreement: Compound Subjects - Exceptions

Exceptions:

1. When the subjects joined by and refer to the same entity, the subject is singular.

• Burger and fries was her preferred snack.

(Burger and fries together form one unit.)

• Drinking and driving is a crime.

(Drinking and driving are not crimes in isolation, only in combination)

• Calculating and plotting the points on a graph is the most time consuming part of the analysis.

• My best friend and wisest counselor is a dog. 

(The two subjects refer to the same animal)

2.  When the elements are joined by and are preceded by the pronoun each and every, they are considered one unit and take a singular verb.

• Each invoice and purchase order has to be approved.

• Each passenger and the crew member was questioned.

• Every man and woman in the country knows who Lata Mangeshkar is.

NOTE: If each follows a compound subject rather than precedes it, the verb is plural.

• The invoices and purchase orders each have to be approved.

Also visit:

Subject Verb Agreement

The Apostrophe

The apostrophe (') has three uses:

1. To form the possessive case of nouns and certain pronouns.
2. To show omission of a letter or letters from words and of a figure or figures from numerals.
3. To indicate the plural of letters, numerals, symbols, and certain abbreviations.

• Add an apostrophe and s to form the possessive case of a noun not ending in s:

women, women's

children, children's

office, office's

horse, horse's

• Add only an apostrophe to form the possessive case of a plural noun already ending in s:

girls, girls'

hero, heroes'

day, days'

student, students'

• Add an apostrophe alone or an apostrophe and s to form the possessive of singular nouns ending in s:

Keats, Keats' (or Keats's)

Sarah Jones, Sarah Jones' (or Jones's)

• In compound nouns add the apostrophe and s to the last element of the expression, the one nearest the object possessed.

somebody else's coat

my mother-in-law's house

the office manager's desk

• Use an apostrophe to indicate that letters or figures have been emitted.

didn't (did not)

he's (he is)

our '88 model car

the summer of '78

Note: It's means "it is" and can never be used correctly for its in the possessive sense.

Before writing its think whether or not you mean "it is".

• Use an apostrophe and s to form the plurals of numerals, letters, and words considered as words.

Sam has trouble making legible 8's.

Don't overuse and's, but's, and for's, in your essay.

Mind your p's and t's.

• Never use an apostrophe in forming the plural of nouns and the possessive case of personal and relative pronouns.

The Smiths (not Smith's) are coming home tomorrow.

Correct: ours, yours, his, hers, its, theirs, whose

Incorrect: our's, ours', your's, yours', his', her's, it's, their's, theirs', who's (unless you mean "who is)

Disinterested vs. Uninterested

Disinterested (adjective) means to have no bias and self-interest, impartial, having no stake in an outcome. A disinterested person is unbiased or impartial.

e.g.

1. Disinterested scientific opinion on fluoride naturally present in the groundwater.

2. Since the judge stands to profit from the sale  of the property, he cannot be considered a disinterested party in the dispute.

Uninterested (adjective) means to have no enthusiasm for something, marked by a lack of interest, not having a financial interest. An uninterested person is indifferent, bored or unconcerned.

e.g. 

1. Sam expressesed his interest in dating Carolina but she is uninterested.

2. Lufthansa's CEO Carsten Spohr has said he is uninterested in buying Etihad's stake in Air Berlin as long as the struggling German carrier's debt situation remains unchanged.

Agree to/ Agree with/ Agree on

Agree to something: Give consent to; accede to
Agree to a suggestion, to agree to an action proposed by someone else.
e.g:
Russia and Spain agree to Cooperate In Energy Sector
We agreed to her suggestion


Agree with someone/ something: get on, be good for, suit; to approve of it
Agree with a person, opinion or policy
e.g:
The copy agrees with the original.
I don’t agree with some of the issues they have raised in the rating rationale


Agree on something:To come to terms, as in the details of a transaction, reach agreement on, settle on
Agree on a matter for decision, to agree on an action that is jointly worked out; have similar feelings about a specific matter
e.g:
Democrat Obama and Republican McCain agree on a range of issues that have divided the parties under Bush
Major EU states agree on tighter regulation

Among vs Between

1) Use between with two items.
e.g:
Emily was standing between Sam and Adam.
Ram divided his property equally between his two children.


2) Between can be used to talk about distance and intervals.
e.g:
I will be at the college between eight and ten.
The distance between wooden studs in new houses in UK is 350mm


3) Between is used before each
e.g:
There seems to be less and less time between each event
Insert a space between each character
There are 9 lines between each centimeter


4) Use among with three or more items.
e.g:
Ram divided his property equally among his three children.
The letter is somewhere among these papers.


5) Use between when something/ someone is between several clearly separated thing/ people. Use among when something/ someone is in a group/ crowd/ mass which we cannot see separately.
e.g:
My villa is between the woods, the river and the village.
Your villa is hidden among the trees.


6) When talking about dividing and sharing things between or among more than two people or groups - can use any of the two
e.g:
Ram divided all his property between/ among his children and grandchildren - both between and among are correct
We shared the work between/ among four of us. - both between and among are correct


7) Generally between is used after difference.
e.g:
What's the difference between a Democrat and a Republican?
There are enormous differences between developing countries in burden of disease , financial resources, educational attainment, and health systems

Usual vs Is Usual

When something is compared to itself, usual is fine.
When something is compared to a subgroup to which it belongs, is usual should be used.

For example:

1. He is nicer than usual

2. He is faster than is usual for any human being.

3. Ram is faster than usual today.

4. Emails are often written in a much formal way than is usual in writing.

5. The sun looks less brightly than usual to-day.

6. The new sports car is heavier than is usual for a performance automobile, but it is exceptional because of its high-powered engine .

7. Successful completion of Grammar course is not easy and requires more planning and preparation than is usual for normal course completion .

8. There was a higher ratio of comedy to drama than is usual in a one hour show.

9. I checked this article more carefully than usual for grammatical errors.

10. The gravel was damper than usual, since it rained heavily yesterday.

11. Today I was busier than usual at work.

GMAT Question example:

The Parthenon is larger than usual for a Doric temple, with eight columns on the short sides and seventeen on the long ones rather than the typical six and thirteen.

(A) usual

(B) is usual

(C) has been usual

(D) one usually is

(E) it is usual

Answer: B

Participles

A participle is a verbal (verb form) which is used as an adjective.

There are three participles that are commonly used as adjective: the present participle(active voice); the past participle (passive voice) and the perfect participle (active voice).

* The present participle always ends in ing
* The past participle usually ends in ed, d, t, n, or en.The past participle of some of the verbs do not have distinctive endings: swum, gone, sung etc.
*The perfect participle is always formed by prefixing the word having to the past participle: having sung, having driven, having seen etc

Consider an example from GMAT Prep SC:

First discovered more than 30 years ago, Lina's sunbird, a four-and-a-half-inch animal found in the Phillippines and that resembles hummingbird, has shimmering metallic colors on its head; a brilliant orange patch, bordered with red tufts, in the center of its breast; and a red eye.

A. found in the Phillippines and that resembles
B. found in the Phillippines and that, resembling
C. found in the Phillippines and resembling
D. that is found in the Phillippines and it resembles
E. that is found in the Phillippines and that, resembling

OA : C

As we know :

Past participle ---- represents
1) a past condition, or
2) the passive voice or
3) a condition done onto the subject

Present participle ---- represents
1) a present condition, or
2) the active voice or
3) a condition performed by the subject

In the given sentence, we can clearly see that for the 1st participial phrase, the action of "found" happened in the past because Lina's sunbird was found more than 30 years ago. Further Lina's sunbird did not do any finding instead they were found by X not mentioned in the sentence so passive voice is right.

The second participial phrase starting with "resembling" implies that the "Lina's sunbird" is still resembling in the present(a present condition), so the use of present participle is better. We cannot say that the Lina's sunbird "resembled" because then this would illogically mean that they no longer resemble the humming bird.

The present and past participles are used to describe the "Lina's sunbird", they act as adjectives..

For more examples click on the link below:
GMAT Sentence Correction Questions

Until vs. As long as

Until can be used both as a preposition and as a conjunction. Until is used to express a point of time in the future

Until (preposition) -

1. Up to the time of: We danced until dawn.
2. Before (a specified time): She can't leave until Friday

Until (conjunction) -

1. Up to the time that: We walked until it got dark.
2. Before: You cannot leave until your work is finished.
3. To the point or extent that: I talked until I was hoarse

As long as means that one thing will occur while another thing is still true

As long as (conjunction) -

1. During the time that: I'll stay as long as you need me.
2. Since: As long as you've offered, I accept.
3. On the condition that: I will cooperate as long as I am notified on time.

Idiom

1. For the period of time that, as in You may keep the book as long as you want, that is, keep it for whatever time you wish to.
2. Also, so long as. Since, because, as in Please pick up some milk as long as you are going to the store, or So long as you're here, you might as well stay for dinner.
3. Also, so long as; just so. Provided that, as in As long as you don't expect it by tomorrow, I'll make the drawing, or So long as sales are greater than returns, the company will make a profit, or You may have another cookie, just so you don't take the last one.

Source: http://www.answers.com

1. In order to express a future idea. after as long as, we use a present tense.
e.g - As long as I live, I shall smoke no more cigarettes
2. Before a number, as long as can be used to suggest great length
e.g - The discussion can last as long as six hours