please explain it to me

[Jolik12]

Dear Teacher,

There is one sentence from yours tests.

It was Paul; I saw him ........ it

to do or doing

I do not understand why correct answer for this question is "to do".
I was taught about 2 tenses in one sentence and one must be continuous.
Please, explain it to me.

Best regards
Jola

[Francois]

I saw him do it => you saw the whole action, from the beginning to the end. Hence you know for sure he did it.
I saw him doing it => you saw a part of the action (you "caught" him doing it), but you don't know if he finished or not.

FRC

[Casiopea]

Well, both are OK, Jolik.

"I saw him doing it" is fine, Jolik, but the example sentence implies the simple present tense:

Simple Sentence: Paul did it.
Cleft Sentence: It was Paul (that did it).
Simple Sentence: I saw Paul do it.
Cleft Sentence: It was Paul (that I saw do it).