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"I saw him to do it."

Is this sentence allowable, with a meaning, "I saw him when he was about to do it" OR "I made him do it by seeing him" = "He had to do it because I was watching him"
Comments  
pructus"I saw him to do it."

Is this sentence allowable, with a meaning, "I saw him when he was about to do it" OR "I made him do it by seeing him" = "He had to do it because I was watching him"

No. You need one of these:

I saw that he was about to do it. [similar to your first meaning]
I saw to it that he did it. / I made sure that he did it. [similar to your second meaning]

CJ
I see....

How about, "I saw him about to do it"?

Is this sentence allowed?
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Yes. That seems OK to me.

CJ
Thanks a lot, CalifJim...
the sentence is equal to the below one:
I saw what he did.
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Hi!

Could you give me a little more?

By "the sentence", you mean "I saw him about to do it." or "I saw him to do it."?
No you can't. You cannot use 'To' after the word saw. "I saw him doing it" will be perfect