• The past perfect is one of the eight basic English verb tenses. It is a compound tense which is formed with the simple past of have (had) as an auxiliary followed by the past participle form of the main verb. In other words:
past perfect = had + past participle.
• The past perfect is used in two important ways:
1. to indicate that a past event happened earlier than another past event
• for example: When Harry got home, Sarah HAD already STARTED to eat supper.
• for example: Harry hired Jill ten years ago. At that time, she HAD just GRADUATED from university.
2. in the ‘if-clause’ of an unreal past conditional
• for example: Jill says that if she HAD KNOWN what sort of person Harry was when he offered her the job, she would not have taken it.
• for example: If Harry HAD FOUND out what Tom was doing, he would have fired him.
i.the labels present perfect and past perfect are logical: the present perfect uses the present tense of have as its auxiliary and the past perfect uses the past tense.
ii. the meaning of the past perfect is not exactly comparable to the meaning of the present perfect: the past perfect can refer to an event that happens at an indefinite time as in the first example above, or to an event that happens at a definite time as in the second example.
iii. the need to use the past perfect can often be used to indicate a “reversal of the normal narrative order” (event, followed by later event, followed by later event etc.) When this is indicated by an adverbial such as "before" as in: Before Harry arrived, Jill had left the past perfect is optional.
iv. the use of the past perfect in past unreal conditionals can be explained in the following way: the simple past has already been used to indicate unreality in present tense unreal conditionals, so in making a past tense unreal conditional it is necessary to resort to the past perfect.