Theory:

The past perfect is a verb tense which is used to show that an action took place once or many times before another point in the past.
Past Perfect Forms: The past perfect is formed using had \(+\) past participle.
 
Questions are indicated by inverting the subject and had.
 
Negatives are made with not.
Example:
1. Statement: You had studied French before you moved to France.
2. Question: Had you studied German before you moved to Germany?
3. Negative: You had not studied English before you moved to London.
Past Perfect Uses
I) Completed Action Before Something in the Past
The past perfect expresses the idea that something occurred before another action in the past. It can also show that something happened before a specific time in the past.
Example:
1. Had Susila ever studied Tamil before she moved to Chennai?
2. Rasi only understood the movie because she had read the book.
3. Kushbu had never been to a play before last night.
II) Duration Before Something in the Past (Non-Continuous Verbs)
With non-continuous verbs and some non-continuous uses of mixed verbs, we use the past perfect for showing that something started in the past and continued up until another action in the past.
Example:
1. We had that Palio car for ten years before it broke down.
2. By the time Alex finished his studies, he had been in Chicago for over ten years.
3. My grandparents felt bad about selling the house because they had owned it for more than twenty years.
Important!
Although the above use of past perfect is normally limited to non-continuous verbs and non-continuous uses of mixed verbs, the words "live," "work," "teach," and "study" are sometimes used in this way even though they are NOT non-continuous verbs.
III) Specific Times with the Past Perfect
Unlike with the present perfect, it is possible to use specific time words or phrases with the past perfect.
Important!
If the past perfect action did occur at a specific time, the simple past could be used instead of the past perfect when "before" or "after" is used in the sentence. The words "before" and "after" actually tell you what happens first, so the past perfect is optional.
Example:
1. She had visited her Russian grandparents once in \(1990\) before she moved in with them in \(2000\).
2. She had never seen a kangaroo before she moved to Australia.
IV) Adverb Placement
The examples below show the placement for grammar adverbs such as: always, only, never, ever, still, just, etc.
Example:
1. You had previously studied Telugu before you moved to Andhra.
2. Had you previously studied Marathi before you moved to Bombay?
V)Active/ Passive: eg:
1. Mani had repaired many bikes before he received his mechanic's licence. Active
2. Many bikes had been repaired by Mani before he received his mechanic's licence. Passive