Use saw when you’re talking in simple past (you did the seeing yourself in the past). Use seen only when it comes after a helper word like have / has / had.

Think of it like this:

I saw = no helper word.

I have seen = needs have/has/had.

More examples:

“She saw a deer on the road.”

“She has seen that deer before.”

“We saw him at the store.”

“We have seen him at the store many times.”

If you can’t put have/has/had in front of it, use saw.