There are a number of regions in the U.S. where most of the population does not use seen and saw in the same format as standard English. It's very difficult for people who grew up this way to switch to correct usages. But let's make it clear what standard English says.
Probably the fast and dirty answer is that you must use "have," "has," or "had" in front of the word "seen." However, "See" and "saw" stand on their own. Never use a form of "have" with either of those.
Here are three official categories. Columns two and three are the only tenses of the verb "to see" that use "seen."
Present tense | Present Perfect tense | Past Perfect tense |
I see | I have seen | I had seen |
you see | you have seen | you had seen |
he/she/it sees | he/she/it has seen | he/she/it had seen |
we see | we have seen | we had seen |
you see | you have seen | you had seen |
they see | they have seen | they had seen |
So just remember to always use a form of "have" with "seen."
P.S. "Seeing" is a whole different ball game. Maybe another time.