Hey Sergii! This sentence sounds very natural and I think it’s fine but if you replaced has eaten with ate it would improve the smoothness. I hope that makes sense!

Hey - in my opinion, if you add 'already' (before eaten), this sentence sounds much more natural :)

‘He has eaten’ tells you that there is a current effect of the past action. ‘He ate’ tells you about a past event. E.g. He ate all the cookies that I cooked this morning. He died immediately because they were laced with poison. (All in the past. We couldn’t say ‘he has eaten’ because he no longer exists.) Use ‘he has eaten’ to convey that there is an effect in the present (there are no more cookies, he is greedy, he is sick etc)

Yes, and it sounds very natural. If you said "He ate all the cookies that I cooked this morning" that, too, is correct and natural and means almost exactly the same thing. I can't think of any example where one would be right and the other would be wrong. Either way, you made the cookies in the morning and there are none left now. There is a subtle difference. "He has eaten all the cookies" refers to an indefinite time, possibly spread out or extended. It is the language we would prefer if he were eating cookies, one by one, all day. "He ate all the cookies I cooked this morning" is the language we would prefer if he ate all of them at once--perhaps at lunchtime.

Hello, Sergii. Your sentence is perfectly conjugated. If you wanted to, you could have also used past simple for both tenses in order to give your phrase a more narrative, completed action meaning.