'I have eaten' is thepresentperfecttense 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》
2
When you ask about something that started in the past but continues to thepresent time, you use thePresentPerfecttense, 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。
3
We use thepresentperfecttense to talk about something that happened at an indefinite time in the past.