Here's a detailed explanation of the past perfect tense:

1. Formation of the Past Perfect Tense:

   • The past perfect tense is formed by using the past tense of the auxiliary verb "to have" (had) followed by the past participle form of the main verb.
   • Example: "She had already finished her homework."

2. Usage of the Past Perfect Tense:

   • Describing an action that happened before another action in the past:
     • Example: "He had already left when I arrived."
   • Indicating an action that was completed before a specific point in the past:
     • Example: "By the time she arrived, I had already eaten."
   • Referring to an event that occurred in the more distant past:
     • Example: "They had visited that city many times before."
   • Expressing regrets or unrealized past events:
     • Example: "I wish I had studied harder."

3. Examples of the Past Perfect Tense:

   • "I had already seen that movie."
   • "She had finished her work before the deadline."
   • "They had traveled to Europe twice before they turned 25."
   • "By the time I woke up, she had already left."
   • "He had never tasted sushi until yesterday."

4. Time Expressions Used with the Past Perfect Tense:

   • Time expressions such as "before," "after," "by the time," "already," "never," "until," "once," and "twice" are commonly used with the past perfect tense.
   • Example: "She had finished cooking by the time we arrived."

5. Negative and Interrogative Forms:

   • To form negative sentences, add "not" after the auxiliary verb "had."
     • Example: "He had not seen the movie before."
   • For interrogative sentences, invert the subject and the auxiliary verb "had."
     • Example: "Had you finished your homework?"

The past perfect tense allows us to describe an action that happened before another past event or a specific point in the past. It helps establish the chronological order of events and provide context to past actions. By using this tense correctly, we can convey a clear sequence of past events in English.