What is the difference between Past Simple and Past Perfect tenses?
When to use which?

We use Past Simple to talk about things we did regularly in the past. And we talk about it in general. For example, I went to school by bus. 
In Past Perfect, we did something in the past but we finished it at a certain time in the past. For example, I had done my homework when my friend came. That means that you finished doing your homework before your friend came.