Third person singular.

To me, it's correct.
It speaks about the whole action that you saw from when the door started to open from the beginning point until it became opened completely.

Consider this: They saw him cry/crying. We heard them argue/arguing.

In your example, "it" is an object, but it may appear to be a subject to you because the form for subject and object are the same for"it"and "you."

We don't want to create subject-verb agreement because"it"is not a subject in this sentence.

Englishmaven Consider this: They saw him cry/crying. We heard them argue/arguing.In your example, "it" is an object, but it may appear to be a subject to you because the form for subject and object are the same for"it"and "you."We don't want to create subject-verb agreement because"it"is not a subject in this sentence.

Nice explanation. Thanks.

Persian LearnerHi.Did you ever reach to open the door at a store or hotel and see it open by itself?Why not 'opens' or 'opened'?Why sort of grammar is this?

There are some verbs, like see, hear, notice, etc., that are called "verbs of perception."

The form that follows is either an "-ing" form or a base verb (no "to" but not changed to match the subje