I have a question: I've learned that one of the differences between the present perfect and the simple past is expressed in the following examples:
I've spent $20 today. (I can still spend money.)
I went to the shops today. (But the shops are now closed... )

But if I want to say that I won't study today because I've already studied too much.
I would say:
I already studied too much/enough today OR
I've already studied too much/enough today.
OR IT DEPENDS

I think you will probably say that the second sentence is correct(because The Present Perfect is being used with the word ALREADY), but WHY it is correct if THE PERSON WON'T STUDY ANYMORE TODAY? If he used the expression TOO MUCH, he won't study today anymore. He wants to let clear that he already studied enough, so the time should be considered finished, don't you agree? It shouldn't have the same rule as the two examples you gave above? Or the word ALREADY changes the explanation, even without convincing me? One more thing: Is there any difference in your answer if I changed TODAY for YESTERDAY, since I think TOO MUCH or ENOUGH implies that I can't study anymore today, even though the day hasn't finished. I mean that I won't study for the words TOO MUCH and ENOUGH imply that I'm full of studying today or that I am too tired.
Thanks in advance,
Ale

I would say that:



I have already studied too much today.

Here's my reasoning:

Although you say you have already done all the studying you are going to do, that doesn't mean that you CAN NO LONGER STUDY, because you still have the ability to study. The first option,
I already studied too much today indeed implies that you have done all the studying you will do, but you will NOT have the ability to study again because, say, you are going to a party and you will not bringing your books along.


I hope that helped!