Is it i saw you yesterday or seen you yesterday?

I saw you yesterday. (past tense of to see)

The verb seen is the past participle of to see, and uses a helping verb.

(I might have seen you yesterday, I could have seen you yesterday)

see - present, saw - past, seen - past participle.

The past participle is used in present perfect sentences:

I have seen the movie three times now.

Or past perfect sentences:

I had seen the movie before.

And other tenses.