Past Perfect

1. Adverbs of time used in this tense

1.1. Already: Occupies the third position

1.2. Still: Occupies the third position, after subject and auxiliar verb "had"

1.3. Yet: It's used in negative and interrogative sentences, it's placed in the end of the sentence

1.4. Ever: Emphasizes some action which has occurred sometimes in the past

2. Third Conditional

2.1. Describe a situation that didn't happen, and to imagine the result of this situation. It's considered unreal

2.1.1. Ex. If I hadn't known him, we wouldn't have been best friends

3. Passive

3.1. Answer the question, "What had been done? or What had been being done?

3.1.1. Ex. The floor had been mop by the time I arrived

4. What is?

4.1. Is a tense used to refers an activity or situation completed before another event or time in the past.

4.1.1. Is more common in written English than spoken English

5. Structure

5.1. There are three ways to express it: Affirmative, negative, question

5.1.1. Aff. I had cooked something for you

5.1.2. Neg. She hadn't studied for the test

5.1.3. Ques. Had he picked her up?

6. Time expressions

6.1. By then, by that time, until then, until that time

6.1.1. By monday at noon had Mehdi finished his class?