The standard distinction
Traditional grammar teaching often connects the present perfect with an unfinished time period or a result that matters now: “Have you eaten yet?”
The simple past is usually tied to a finished past event: “Did you eat at noon?”
What happens in American conversation?
In casual American speech, the simple past is frequently used with yet and already, especially when the meaning is obvious from context.
Also standard: “Have you eaten yet?”
Neither form is difficult to understand. The choice is partly about register and regional usage, not only a grammar table.
What should a learner do?
Learn the standard distinction so your grammar is reliable. At the same time, train your ear to recognize the forms Americans actually use in conversation.
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