The verb in the object complement is the bare infinitive.
I heard her object sing. complement
I saw him object run. complement
You have seen it object happen.complement
When we put the video in super slow-motion, we can see the hummingbird's wings beat.
The objects are not in nominative case but objective case (her, him).
This construction is not to be confused with a relative clause:
You have seen that it happens.