Choosing whether to use the Present Perfect or Past Simple usually depends on whether a definite or indefinite time is used. If a definite past time is used, use the Past Simple, and if there is no time given use the Present Perfect. Examples below are split into two groups, single events and multiple events.

Single Events

- I went to America in March. - This sentence has a definite past time, so the Past Simple is used.
- I've been to America. - Here the time is not important, what is important is the fact I went.

- I read this book last week. - Last week I started and finished this book.
- I've read this book. - Here also the time is not important, what is important is the fact I've already read the book.

The Present Perfect can however be used with a period of time that isn't finished yet.

For example:
- I've been to the doctor today. - Today is not finished, so this is correct.
- I went to the doctor today. - This is also correct, a statement about a past action.
- I've been to the doctor yesterday. - This is incorrect, yesterday is a finished period of time that does not continue into the present so we cannot use the present perfect here.

Multiple Events

The Past Simple and Present Perfect can similarly be used for multiple events.

For example:
- I went to America three times last year. - Here the time is important.
- I've been to America three times. - Here the time is not important, it's the fact I've been that is important.

AS above you can also use the present perfect for multiple events when the time is not finished.

For exmple:
- I've been to America three times this year. - This year isn't finished yet, so this is correct.
- I've been to America three times last year. - Incorrect, last year is a finished time period, so you can't use the Present Perfect.

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