Dear Miss EW:

You wrote:
> Dear Sir,
> I got a sentence from my professor ” I think him (to be) a good man’. He
> said that this sentence is correct in some sense. Can you help me out by
> giving me the explanation why this sentence is correct?
>
> Thanks a lot!

This is perfectly good English. It is perhaps more commonly heard in England than America. It would be the same as saying “I think of him as a good man.”

To parse the sentence: “I” is the subject; “think” is the verb; “him” is the direct object; “man” is the object complement; “a” and “good” modify man.

If you insert “to be,” then “to be” becomes the direct object (a noun infinitive); “man” becomes the predicate nominative of the infinitive; and “him” becomes the subject of the infinitive.

I hope this helps.