That's a good question. If you have a simple sequence of past events, then each event is expressed in the simple past. "I did my homework. Then I ate my dinner. Then I went to see my friend" This is an example of the use of the past perfect. "By the time I was ready to eat my dinner, I had already done my homework." In this second case, we are learning more about a specific time in the past - the time when you were ready for dinner. The past perfect is used toi tells us what happened before this specific time - you had done your homework. I recommend you do a few grammar exercises on the past perfect to consolidate your knowledge, for example, from www.englishpage.com

PS... I don't think it's merited for you guys to answer...I just don't know how I can be polite to ask this in English... I am so sorry for this!