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。
'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.
它们是用作现在完成时和过去完成时的助动词。
They are used as auxiliaries to the present and past perfect tenses.
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