There are many rules and examples to explain when it is correct to use the Present Perfect, and when to use Past Simple.
Perhaps the simplest way to remember the main difference is to think in terms of an ‘open’ period of time and a ‘closed’ period of time.
Generally we use the Present Perfect if we are talking about an action taking place in an open period of time. This means a period of time that is not finished, includes the present and possibly the future.
“I have lived in this house all my life.”
From this sentence we understand the person’s life as an open period of time (the person is still living). So, they have lived in that house until now and may continue living there in the future. (Or not).
However, if the sentence refers to a closed period of time, in other words, a specific period in the past that is finished, we would use the Past Simple.
“I lived in this house all my childhood.”
If the person speaking is an adult, then the period of time (their childhood) is closed since he or she is not a child anymore. This is why we use the Past Simple.
Some course books and teachers talk about actions that are completed or not completed. This can be confusing in sentences like these:
“I have written ten text messages this morning.”
“I wrote ten text messages this morning.”
The difference between the two is not related to the action being completed or not. (In both cases the action of writing ten messages is completed.) The difference is that in the first sentence we understand that the period of time (this morning) is still open. In other words, it is still morning, and it is possible for that person to write more messages in that period.
In the second sentence, we understand that the period of time is closed. (Now, it is afternoon.)
This grammar tip can help you remember and/or understand other rules of Present Perfect vs. Past Simple such as:
Present Perfect is used for actions that continue in the present.
In the sentence, “I have worked here for fifteen years.” the 15 years is referring to an open period which is not finished and could go on to become 16 years, 17 years, etc.
However, in the sentence “I worked here for fifteen years.” the 15 years is referring to a closed period. So, it is clear that the person speaking does not work there now.
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