'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》

To grasp the usage of the present perfect tense.

掌握现在完成时态的用法。

youdao

He wants to know what situation calls for use of the perfect tense.

他想知道在什么情况下需要用完成时态。

youdao

Adding to the stress is the complexity of the grammar, especially the far-from-perfect spelling system hellish past tense riddled with irregularities.

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