help,what's the deffirent between the simple past & the past perfect?

hi everybody,

i’ve a problem to discrimination between simple past and the past perfect.

what is the deffirent between them? & when we use each one of them?

simple past:
subject + past verb + … + .

past perfect:
subject + had + past participle + … + .

past perfect is used to show you which of two events happened first. Imagine that two things happened in the past:

I went to see the movie. + We discussed the movie in class. =

I went to see the movie. We had discussed it in class

thank you very much for the explain