The past perfect tense is a verb tense used to describe an action that was completed before another specific point in the past. It is formed by using the past tense of the auxiliary verb "have" (had) and the past participle of the main verb. This tense is often employed to clarify the sequence of events in the past and to emphasize the completion of an action before another event or time reference.

The structure of the past perfect tense involves the use of the past tense of the auxiliary verb "have" (had) and the past participle of the main verb. Here's the basic structure:

[ {Subject} + {had} +{past participle of the main verb} ]

Here are a few examples to illustrate the structure:

1. Positive Form:

    She had finished her work before the meeting.

2. Negative Form:

    They had not completed the assignment by the deadline.

3. Interrogative Form:

    Had you ever visited that place before last summer?

4. Negative Interrogative Form:

    Hadn't they already left when you arrived?

Here are examples to illustrate the past perfect tense:

1. She had already finished her homework before the party began.

    In this sentence, the action of finishing homework (past perfect) is completed before the action of the party beginning (simple past).

2. By the time we arrived, they had already left for the airport.

    The action of leaving for the airport (past perfect) is completed before the action of our arrival (simple past).

3. They realized they had forgotten to buy groceries when they got home.

    The realization of forgetting to buy groceries (past perfect) occurs after the action of getting home (simple past).

4. He had studied French for several years before he visited Paris.

    The action of studying French (past perfect) is completed before the action of visiting Paris (simple past).

5. The team had scored three goals before the opponents even had a chance to respond.

    The scoring of three goals (past perfect) precedes the opponents having a chance to respond (simple past).

In each case, "had" is the past tense of the auxiliary verb "have," and it is followed by the past participle of the main verb, indicating an action completed before a specific point in the past.