I am having trouble with seen and saw.
When do you know what one to use?
The verb forms of "see" are

see (infinitive)
saw (past)
seeing (present participle)
seen (past participle)

Present tense
I see, you see, he sees, we see, you see, they see
Past tense:
I saw, you saw, he saw, we saw, they saw
I saw my friends yesterday at school.

Future tense:
I will see,....

Present perfect tense
I have seen, you have seen, he has seen, we have seen...
Example: I have seen the Statue of Liberty six times.

Past perfect tense:
I had seen, you had seen,....
Example I had seen the movie at least five times in theaters before it was released on DVD.

Passive forms:
He is seen once a week by the doctor. (present passive)
He was seen last week by a specialist. (past passive)
He will be seen next week by a new doctor.