Re: have seen
Hi, and welcome to Using English.
"Have seen" is present perfect. (The "helping" verb, "to have" is in the present tense.)
"Had seen" is past perfect. (The helping verb is in the past tense.)
A: Do you want to see the new Sherlock Holmes movie?
B: Could we pick another? I have seen that one already.
-- It relates to an action in the past that connects to our present situation.
I was halfway through a movie on demand when I realized I had seen it before.
-- It's a past action that has another past action after it. I had seen it, then I realized it, both in the past.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.