'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。
To grasp the usage of the tenses, including the present, past, future tense and continue, perfect tense.
掌握现在时，过去时， 未来时， 进行时， 完成时的各种用法。
So you learn the past tense and then two or three weeks later, oh now you are learning the present perfect, and then a few weeks later, you are learning the future, that's crazy.
你学习了过去时，然后两到三周后你开始学习现在完成时，几周以后你又开始学习将来时，这简直太疯狂了。
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