That is a very large question. Have you tried Googling the two tenses for information?
Student or Learner
Can anyone explain how and when to use perfect tenses?
What is the difference of past perfect tense to past tense?
***** NOT A TEACHER *****
As I understand it, the past perfect = you did something before you did something.
"Mona had studied English (in 2005 - 2008) before she came to the United States (in 2009).
"I had eaten dinner (from 6 p.m. to 6:30 p.m.) before you telephoned me (at 6:35 p.m.).
BUT in ordinary conversation, many native speakers do not use the past perfect if the meaning is clear: "Mona studied English before she came to the States" and "I ate before you telephoned me."
In my opinion, since you are a learner, you should always use the past perfect when it is required by the rules of standard English. When you become fluent in the language, then you will know when you can "break" the rule.
Thank you for the information.
It's really a great help!