When two actions take place in the past, we use both past simple and past perfect tenses in one sentence. However, we use past perfect tense to talk about the action that happened first and the simple past tense for the action that happened last:
When I reached the station, the train left. ✗ (In this sentence, you’d wonder whether I reached the station before the train left or after it did).
Now study this sentence:
When I reached the station, the train had left. √ (things now clear: the train had already left before I reached the station).
The past perfect tense is formed with had + past participle of the given verb.
Now we put it another way:
I had reached the station before the train left. (here, you can make out that I was not late and was able to catch the train)
Notice that the past perfect tense is used for the action which takes place first in the past and the simple past tense for the action that happened next.
Remember that we use the time expressions – for, since, already and yet in the past perfect simple in the same way as in the present perfect simple.
We also use after, as soon as, the moment that, until before using the past perfect simple:
After she had boarded the plane for Istanbul, I found her handbag in the hallway. (I didn’t find her handbag until she had boarded the plane).
We use before, when, by the time before the past simple:
Before he could understand anything, police had taken him into custody and handcuffed him.
Now it is clear that we use the past perfect tense to show that one action happened before the other in the same sentence that is described by verb in the simple past tense.
When using the Past Perfect is not desirable
We don’t use the past perfect when we don’t have to convey some sequence of events. If somebody asks you what you did after you found the wallet and your answer might confuse them if you said:
I had deposited it with the police.
One would likely be wondering what happened next because using the past perfect entails that your action of depositing the wallet happened before something else happened, but you don’t clear what that ‘something else’ was. Though this ‘something else’ doesn’t always have to be mentioned, the context required to make it clear. Here, there’s seemingly no context, so the past perfect is undesirable.
Some more examples:
When she stopped storytelling, everyone fell asleep. (= everyone fell asleep after she stopped storytelling)
When she stopped storytelling, everyone had fallen asleep. (= everyone fell asleep before she stopped storytelling)
She got up when the maid knocked at the door. (= the maid knocked at the door and then she got up)
She had gone out to buy groceries when the maid knocked at the door. (= She went out to buy groceries and then the maid knocked at the door)
After Jane had finished reading novel, she switched off the light.
We also use past perfect when we say the things didn’t happen the way we wanted them to:
I had wanted to see this movie before it was pulled from cinemas, but I had an important assignment.