Past Perfect is used when we want to say that an event happened before another event. Example: Tom left the party at 8PM. Jane arrived at the party at 9 PM. --> When Jane arrived at the party, Tom had already gone. When explaining Past Perfect to my students, I tell them it's like the Present Perfect, told later. --> Present Perfect: Who is that man? I have never seen him before. At the moment of speaking, you see that man, so the "I haven't seen him" period is over. Thus, when you talk about this a few days/months/years later, you can say that until that moment, you hadn't seen him. Past Perfect: I didn't know who that man was. I hadn't seen him before. If you are waiting for the bus, but it doesn't come, you can say: I have been waiting for the bus for an hour. If you tell your friend about this a few days later, you can say: When the bus came, I had been waiting for an hour. Hope this helps.

Hi! It's quite a big topic to explain just on here, unfortunately. The past perfect is when we construct our sentences like this: (had + past participle) Examples: You had studied English before you moved to New York. Had you studied English before you moved to New York? You had not studied English before you moved to New York.