Why must you put present tense behind the past tense for example He SAW the monkey CLIMB last night.?

It is the same as , ''Did you eat?''

That is the way imperfect tense works in English. You don't have to do it that way if you're not discussing things that happened in the past.

You can say:

He sees the monkey climbed last night.

But that means that right now, he is seeing evidence of events that occurred in the past (such as the monkey destroyed something by climbing and he's looking at it).

But, when using past tense, only the first verb is past tense then one is discussing events that happened continuously in the past.