Generally there is a lot of confusion between using the simple past and the past perfect tense. The past perfect tense always has a verb in the past participle form that is followed by ‘had’.
The past perfect tense is used when you want to say something happened before another past action. If two actions happened, the action that happened first comes in the past perfect, and the action that happened second, takes the simple past form.
Example 01: When I reached the station, the train had started. (‘had started’ is the past perfect tense. This shows, first the train started and then I reached.)
Example 02: Jane had read a lot before she went to the zoo. (‘had read’ is the past perfect to show that it happened first.
Example 03: Before the installation of the new software, productivity had been low.
Example 04: After I had had my dinner, I completed my work. (first I had eaten my dinner)