The past perfect tense is used to express events that took place before a certain point in the past. It is formed by using the auxiliary verb snake and the past participle. For example:
write → had written
sleep → had slept
It is usually not found alone in a sentence, but together with another past tense expressing an event that occurred between another, earlier event and the present:
John suddenly realised that he had left his laptop on the train.
In the above example, we have two past tenses to distinguish which event happened earlier. John could only have remembered that he had left his laptop on the train after it happened. In the case of this sentence, it can be logically deduced.
If the sentence is less clear, the presence of conjunctions such as before, until, after, which indicate the sequence of events, is helpful.
The past perfect tense is also sometimes called the Pluperfect.
The following example will show how the use of the past perfect tense changes the meaning of the sentence:
Jane cooked breakfast when we got up.
If the past perfect tense were in the sentence (“had cooked”), it would mean that Jane had cooked before they got up. The use of the past simple tense indicates that Jane cooked after they got up.