Past perfect tense is used to indicate that one action occurred before another action in the past. In other words, past perfect tense indicates the first of the two actions.*

For example,

When I woke up this morning, my roommate had left already.
After I had eaten my dinner, I went to see a movie.
Before I arrived at the theater, the movie had already begun.

Sometimes, when the meaning is clear from context, the simple past tense can be used.

After I had gone shopping, I stopped at the health spa.
After I went shopping, I stopped at the health spa.

The most common error with the past perfect is using it where it does not belong.

When I was young, I had been a cowboy. When I was young, I was a cowboy.

(Incorrect—no reference to other events) (Correct)

Yesterday the Johnsons had opened their new business. Yesterday the Johnsons opened their new business.

(Incorrect) (Correct)

*In some very unusual cases, the past perfect tense can refer to the second past action.

Before I had finished my homework, the telephone rang.
(The telephone interrupted me before I finished my homework.)