What are the difference between the following sentence?
- I ate apples.
- I have eaten apples.
When should we use simple past tense? When should we use perfect past tense?
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Let me use two different sentences to explain. 'I decided to get a degree in computer science when I was in university.' 'I have decided to reply to your question.'
The first, in the past tense, refers to an act that took place in the past and is not directly relevant to what's going on at this present time. If you were writing a history you would use the past tense a great deal.
The second, in the present perfect tense, refers to an act that has been completed and has an effect on the present time.
"I ate apples." implies one or more specific instances of eating apples (e.g., answering the question: "which fruit did you eat last Saturday at brunch"), or, the admission of a truth about yourself as an eater of apples in a specific instance (i.e., the affirmative answer to a "Yes or No" question about a specific instance).
"I have eaten apples." implies a general truth, unrelated to any specific time, place, context, (e.g., answering the question, "which fruits have you eaten in the course of your adult life?") or the admission of a truth about yourself as an eater of apples at any time (i.e., the affirmative answer to a Y or N question about your past in general).
Use simple past tense when you are referring to context: the what, where, when and why of specific events, actions, etc. (Those specific events are implied from the tense when you don't actually include them.) For example, in the above phrase: "I ate apples ...", "<... context>" may be "on last Saturday" or "but not bananas on last Saturday" or "but not bananas on Saturday because the doctor told me that morning that bananas would constipate me."
Use past perfect tense when you are referring to something more general, e.g., "I have eaten apples ...", "<...context>" may be "on a few Saturdays in the past" or "instead of bananas when I have been constipated in the past."
Choosing one tense versus the other, when you are not including any other contextual information, adds clarity to what you are saying, such as specific instances of eating apples, or generally eating apples in certain situations.
I found I understood these two tenses much much better after I learned Spanish and we had to memorize the reasons for using one tense versus the other. In English it is not a strict rule, the way it is in Spanish, but I think the "rule of clear speech" controls nonetheless.