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-The present perfect is used when the time period has NOT finished:
"I have eaten two ice-creams this week" (the week is not finished)
-The simple past is used when the time period HAS finished:
"I ate three ice-creams last week" (last week is finished)
-The present perfect is often used when giving recent news:
"Jack has broken his arm again" (recent news)
-The simple past is used when giving older information:
"Jack broke his arm last year"-The present perfect is used when the time is not specific:
"I have already read that book" (I don't remember when)
-The simple past is used when the time is clear:
"I read that book last month"-The present perfect is used with for and since, when the actions have not finished yet:
"I have lived in this city since 2000" (I still live here)
-The simple past is used with for and since, when the actions have already finished:
"I lived in this city for a long time" (I don't live here now)
To practise these structures, click on the title of the post and watch this presentation.