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

Let's start with an explanation of what is Past Simple and Past Perfect tenses. In the beginning, there are times, which we used to describe what we did in the past. We use Past Simple when we want to describe a simple action in the past, for example, "I saw a dog yesterday". Maybe it happened only one time or maybe never, that is why "I didn't see a dog". We use Past Perfect when we want to describe an action which was in the past until the specific time or moment, for example, "He had done his homework before his mother came". Usually, we use Past Simple and Past Perfect together, but Past Perfect will be in the first part of the sentence and in the second part - Past Simple.