'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。
The present and past perfect tenses both belong to the perfect tense.
现在完成时和过去完成时都属于完成时态。
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