Another way to approach present perfect is by the different ideas you can express with it.

My grammar book in my high school days told me that PP can represent four things:

1. completion:  I have done my homework. (I have done it to completion)

2. result:  I have botched it.  (there is something botched now as a result)

3. continuation:  I have known him for 10 years.  (I knew him continuously for ten years)

4. experience: I have been to China (I have had the experience of going to China).

A PP sentence usually has one or more of these senses.

If you just can't understand present perfect, thinking of it in terms of these four classes of meanings might help too.

Hello Mathius, both sentences are correct grammatically. But the difference, which I didn't make clear, is the use of the negative, haven't, is a reprimand, something the person shouldn't have done.

This can be a silly doubt, but I'd like to know if in the following sentence:

“You haven't already eaten all those chocolates!” (There were some chocolates and the other person finished them before expected.)

wouldn't it be: 

“You have already eaten all those chocolates!” (There were some chocolates and the other person finished them before expected.)?

Nice article, I have read over and over again since yesterday. Thanks for talking a while and write this useful article, about one of the most difficult tenses (in my point of view) to understand related to english language.

Best regards from Argentina!

Well, French, Italian and German don't. Those are the languages I have studied. Perhaps other languages don't either.