Several questions that trouble me ...

1. After "If I", we must make the next word past tense ? Like If I SAT next to you, or If I SIT next to you ?

2. Why doesn't have instead of doesn't has ? Since doesn't means singular and has is categorized as singular ? He doesn't have instead of doesn't has ?

3. Why "let it go" instead of "let it goes" ? I thought "it" stands for singular ? And they for plural ..

1.  After "If I" you don't have to make the next word past tense, no.

2.  See Re: Have or has.

3.  In Let it go, go is an infinitive without to -- a bare infinitive -- invariable -- not an inflected form.

Let it go.  Let us go.  Let them go.  Let Jack see it.  Let the children see it.

let, make, have, and help all follow the same pattern:  Make him go.  Have her fix it.   Help me do this. 

CJ 

There are so many terms for the same thing, I just thought that pure infinitive was another term for bare infinitive.  Pretty good for a made-up term!  

I think there are other terms for it, too, but they are slipping out of my sieve-like mind just now.

CJ