Past Simple is used for an action that happened in the past and is finished, it cannot happen again. For example: "Did you see the film yesterday?" - yesterday is over, so you can't watch the film yesterday again. If you say "Have you ever seen this film?", we use the Present Perfect, because there isn't a determined time period for watching the film, so you can still watch it.
Similarly: "I was in London two years ago" refers to a past time period, so it's over, you can't go to London two years ago again. But if you say "I have been to London three times", it means you can still go there, there is no determined time period.
Or: "Jane Austen wrote six books." - she is dead, she can't write any more --> "Joanne Rowling has written several books." - she is alive, so she can write more.
The past simple is used for time periods that are finished : last week, last month, 2012, ten years ago, when I was a child, before the second world war, in the 19th century and so on.
For example, you can't say 'I have been to Rome last month' or 'I have been to Rome two weeks ago'. These are finished time periods, so you have to use the past simple 'I went to Rome last month/ two weeks ago.'
To say something that happened in the past and it's finished.
In addition to what others have said, the simple past is used to describe one or more single events (or something considered to be so).
1. simple past - I watched TV yesterday
past progressive - I was watching TV yesterday when the phone rang.
2. simple past - I ate at McDonald's. (once)
simple past - I ate at McDonald's twice in February.
imperfect - I used to eat at McDonald's. (it was a customary thing that happened on regular basis)
3. simple past - I went to Moscow twice in my previous life. (not very useful for most people, but this is how the tenses work)
present perfect - I have been to Moscow twice. (referring to this lifetime)