I'm confused by the use of these two tenses "I saw him TAKE it" and "I thought he KNEW it" Why I saw him + infinity form verb but I thought he + past tense??
These are just the constructions that those particular verbs happen to take.
"Think" takes a "that" clause, which is finite and therefore tensed (the "that" is often omitted):
I thought [that] he took it.
"See" (in this sense) takes a non-finite clause (and if its subject is a pronoun, it is in the oblique case):
I saw him take it.
You can say "I saw [that] he took it", but it has a rather different meaning: something like "I saw something that led me to conclude that he took it, but I didn't necessarily see him actually take it".