“Saw” is the past tense of “see”. “Seen” is the participle form of the verb.
So:
I see → present tense
I will see → future tense
I saw → past tense
I have seen → present participle
I had seen → past participle
I will have seen → future participle
There are more tenses, but those are the most common, other than the various conditionals: should have / would have / could have, etc.
You can quote Roberta Flack for correct usage: “The first time ever I saw your face…” She said it correctly.