when do I use "had"?

[Timidboy]

i was told that the word "had" is kind of past-past tense.
but i am still not very sure when is the right time to use it.

also, is it possible to have a past tense after had? would it become past-past-past tense?!?! for instance, had finshed

so which should be the correct one?
1. she had finshed her homework this morning.
2. she had finsh her homework this morning.

Thank you.

edit:
i did the quiz and i just realised that "had finshed" is a Past Perfect Simple. then my question now is, when do i use Past Perfect Simple?

thank you

[Thanos]

"She had finish her homework this morning" doesn't make sense.
"She had to finish her homework this morning" and your first answer does.
"She had better finish her homework" makes sense but the "this morning" phrase makes it incongruent as it means it's no longer morning and the previous quoted sentence is a present threat ^^

I googled for 'past perfect' and found this among others : http://www.englishpage.com/verbpage/pastperfect.html
Maybe it can help us both