Teorija

Study the information about the past perfect:
 
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.Tony knew Istanbul so well because he had visited the city several times.
Duration before something in the past
We use the past perfect with non-continuous verbs* to show that something started in the past and continued up until another action in the past.
By the time Alex finished his studies, he had been in London for over eight years.
Specific times with the past perfect
It is possible to use specific time words or phrases with the past perfect. Although this is possible, it is usually not necessary.She had visited her Japanese relatives once in 1993 before she moved in with them in 1996.
Past perfect and past simple
If the past perfect action did not occur at a specific time, the simple past can 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. For this reason, both sentences in the example are correct.
She had visited her Japanese relatives once in 1993 before she moved in with them in 1996.

She visited her Japanese relatives once in 1993 before she moved in with them in 1996.
Past perfect, not past simple
If past perfect is referring to a lack of experience rather than an action at a specific time, the past perfect is not optional. For this reason, the simple past cannot be used.She had never seen a bear before she moved to Alaska.
 
*Non-continuous verbs usually indicate actions you cannot see somebody doing. They include:
 
Abstract verbs                
to be, to want, to cost, to seem, to need, to care, to contain, to owe, to exist, etc.
Possession verbs
to possess, to own, to belong, etc.
Emotion verbs
to like, to love, to hate, to dislike, to fear, to envy, to mind, etc.
 
Svarīgi!
Don't mix up the present perfect and the past perfect.
Both tenses talk about something that happened before a point in time.
 
In the present perfect, our reference point is in the present.
 
In the past perfect, our reference point is in the past.
 
I have lived in this city for six months. (the present perfect) – an action that started in the past and continues in the present.
 
When I arrived at work, I discovered that I had left my keys at home. (the past perfect) – an action that happened before a time in the past.
 
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Atsauce:
www.espressonenglish.net
www.linguisticsgirl.com