The auxiliary verb "do" - followed by infinitives without "to". It has several uses, one of them -- Ellipsis
In cases where an auxiliary verb is used instead of a whole verb phrase, "do" is common in affirmative clause, questions and negatives. e.g
a). She doesn't like singing , but I do.
b). You saw Ram, didn't you?
c). Emil thinks there's something wrong with Ann, and so do I.
An auxiliary verb combines with another verb to help form the tense, mood, voice, or condition of the verb it combines with.
The verbs to have, to be, to do, will, shall, would, should, can, may, might, and could are the common auxiliary verbs in English.
Auxiliary verbs are sometimes called helping verbs.
Ellipsis - We often leave out words to avoid repetition, or in other cases when the meaning can be understood without them. This is called ellipsis.