Describe the rules for forming the past perfect tense.

The past perfect tense is used to describe an action that occurred before another action in the past. It is formed by using the auxiliary verb "had" followed by the past participle of the main verb.

The rules for forming the past perfect tense are as follows:

1. Auxiliary verb "had": The past perfect tense always requires the use of the auxiliary verb "had" as the helping verb. This remains constant for all subjects, whether singular or plural.

2. Past participle: The main verb in the past perfect tense is always in its past participle form. Regular verbs form their past participle by adding "-ed" to the base form of the verb. For example, "walk" becomes "walked." Irregular verbs, on the other hand, have their own unique past participle forms. For example, "go" becomes "gone."

3. Placement of "had": The auxiliary verb "had" is placed before the past participle of the main verb. For example, "She had finished her homework before going to bed."

4. Time markers: The past perfect tense is often used with time markers such as "before," "after," "already," "just," "never," "once," "until," and "when." These time markers help to establish the sequence of events in the past. For example, "I had already eaten dinner when she arrived."

5. Use of conjunctions: The past perfect tense can also be used with conjunctions such as "before," "after," "when," and "until" to connect two actions in the past. For example, "He had studied for hours before he took the exam."

6. Negative form: To form the negative form of the past perfect tense, the word "not" is placed between the auxiliary verb "had" and the past participle of the main verb. For example, "They had not seen each other for years."

7. Interrogative form: To form a question in the past perfect tense, the auxiliary verb "had" is placed before the subject, and the main verb is used in its base form. For example, "Had you finished your work before the meeting?"

In summary, the past perfect tense is formed by using the auxiliary verb "had" followed by the past participle of the main verb. It is used to describe an action that occurred before another action in the past. Time markers and conjunctions are often used to indicate the sequence of events.