See Someone Do vs See Someone Doing: Perception Verbs and Viewpoint

I saw her cross the road and I saw her crossing the road can describe the same real-world event, but the speaker frames it differently.

Bare infinitive presents the event as a whole

We heard the door close. The closing is viewed as a bounded event. This does not necessarily mean the observer watched every millisecond; it means the grammar packages the event as complete.

-ing puts the observer inside the activity

We heard the children singing. The emphasis is on the activity in progress. The speaker does not present a natural endpoint.

Passive forms change the pattern

After a passive perception verb, English usually restores to: She was seen to enter the building. This is one place where the patterns differ from the active They saw her enter.

If bare-infinitive patterns are still unfamiliar, review verb + object patterns. For the broader contrast between infinitives and -ing forms, see gerunds vs infinitives.

Advanced control comes from viewpoint: ask whether you are presenting the event as a complete unit or placing the listener inside an unfolding scene.