ate vs. have eaten

What is the difference between these two sentences?

I ate lunch.
I have eaten lunch.

Please don't think about the grammar terms (names) for the sentences. Just think about TIME. For a lot of grammar patterns, WHEN is the important nuance. I think it is best to draw a picture. It is better not to think of grammar words or translations into Japanese because then you will just think about study and you might not get the FEELING of the communication.

Here is my picture for the two sentences:

Click on the picture to make it bigger.

So let's look at the questions again ...
First of all, let's think that usual lunchtime begins about 11am and ends about 2pm.
Now it is 4pm. Lunchtime is finished. It would be a little strange to eat lunch at 4pm. So our question is simple past:
"Did you eat lunch?" Answer: "Yes, I ate a sandwich." 

Much grammar is like this because often native speakers use the simplest and easiest way. I think it is the same in Japanese.

But in Question 2, lunchtime hasn't finished. It is 1:30pm so it is possible that you have already eaten and it is possible that you haven't eaten yet. We often use grammar to show this longer period of time.

So my nuance in this question is that I understand maybe you have already eaten, but because it is still lunchtime, maybe you haven't eaten yet. In Japanese you use words like "mo/mada".

There is no correct and incorrect. It is case by case depending on the situation and the time now and the usual time for doing something.


Did you eat lunch? (I think you already ate it and I am just checking.)

Have you eaten lunch? (Maybe you ate it, maybe you haven't eaten it yet.)

What will you eat for lunch? (I think you haven't eaten it and you haven't decided what to eat.)
What are you going to eat for lunch?  (I think you haven't eaten it and you have decided what to eat.)

Your answer can be different to the question:
What did you eat for lunch?   I haven't eaten lunch yet.
Have you eaten lunch?   Yes, I ate something at 11am.


If we are talking about experiences, the start time is the time when we were born:

Have you been to Hawaii? 
(From the time you were born, up to now, have you had the experience of going to Hawaii?)

But if that thing is not possible any longer because there is no future (it is not possible in the future), then time is cut so please use simple past:

Did you meet Princess Diana? 
(She passed away so you cannot use any nuance of future.)


Often there are two choices, and your choice shows the nuance:

I studied that book. (I finished it and I won't study it again.)
I have studied that book. (I recently studied it/I will study it again.)