'I have eaten' is the present perfect tense of the verb 'to eat', 'I had eaten' is the past perfect and 'I will have eaten' is the future perfect.

I have eaten是动词eat的现在完成时；I had eaten是过去完成时；I will have eaten是将来完成时。

《牛津词典》

When you ask about something that started in the past but continues to the present time, you use the Present Perfect tense, e.g.: I’ve been there for 5 years / I’ve been there for 5 years.

当你谈论在过去开始但是一直持续到现在的某事时，用现在完成时态，例如：I’ve been there for 5 years/我在那里已经5年了。

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We use the present perfect tense to talk about something that happened at an indefinite time in the past.

我们使用现在完成时谈论发生在过去某个不确定的时间的事情。

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