We use the Present Perfect to say that an action happened at an unspecified time before now. The exact time is not important. You CANNOT use the Present Perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in Japan, at that moment, that day, one day, etc. We CAN use the Present Perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, etc.
Examples:
I have seen that movie twenty times.
I think I have met him once before.
There have been many earthquakes in California.
People have traveled to the Moon.
People have not traveled to Mars.
Have you read the book yet?
Nobody has ever climbed that mountain.
A: Has there ever been a war in the United States?
B: Yes, there has been a war in the United States.
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.
Examples:
I had never seen such a beautiful beach before I went to Kauai.
I did not have any money because I had lost my wallet.
Tony knew Istanbul so well because he had visited the city several times.
Had Susan ever studied Thai before she moved to Thailand?
She only understood the movie because she had read the book.
Kristine had never been to an opera before last night.
We were not able to get a hotel room because we had not booked in advance.
A: Had you ever visited the U.S. before your trip in 2006?
B: Yes, I had been to the U.S. once before.
The Past Indefinite Tense denotes actions or happenings that took place in the past.
Clarification:
1. My friend has lived in the USA since 1974.
2. My friend lived in the USA.
The first sentence shows that my friend is still in the USA. The second sentence shows that my friend is not in the USA.
Pay Attention: In Active Voice, the second form of the verbs (educated, worshipped, won, lived, constructed, etc.) is always used. In Passive Voice, be (was, were) + third form of the verbs is used.
