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I am having trouble with seen and saw.
When do you know what one to use?
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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
Example:
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.