The Simple Past Tense And The Present Perfect Tense

The present perfect is used when the time period has NOT finished:

I have seen three movies this week.
(This week has not finished yet.)

The simple past is used when the time period HAS finished:

I saw three movies last week.
(Last week has finished.)

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The present perfect is often used when giving recent news:

Martin has crashed his car again.
(This is new information.)

The simple past is used when giving older information:

Martin crashed his car last year.
(This is old information.)

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The present perfect is used when the time is not specific:

I have seen that movie already.
(We don't know when.)

The simple past is used when the time is clear:

I saw that movie on Thursday.
(We know exactly when.)

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The present perfect is used with for and since, when the actions have not finished yet:

I have lived in Victoria for five years.
(I still live in Victoria.)

The simple past is used with for and since, when the actions have already finished:

I lived in Victoria for five years.
(I don't live in Victoria now.)

Reference:

Study Zone, English Language Centre, University of Victoria