• 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.)

• 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.)

• 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.)

• 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.)