GRAMÁTICA: Present Perfect Simple Tense

source: http://www.learnenglish.de/grammar/tensepresperfsim.htm

English Grammar

Present Perfect Simple Tense

Overview | Simple Present | Simple Future | Simple Past
Present Continuous | Continuous Future | Past Continuous
Present Perfect Future | Present Perfect Continuous
Past Perfect Simple | Past Perfect Continuous
The Future

The present perfect simple tense is used to talk about a past time, which has very strong meaning for the present.

Present Perfect Simple Timeline

For example:
Q) Where's Jane?
A) She has gone out. She should be back in an hour.
We form the present perfect simple by using the auxilliary verb have/has and the -ed form of the regular verb (the past participle) irregular verb forms have to be learned:

Statements +	Statements -	Questions	Short answer +	Short answer -
I've worked.	I haven't worked.	Have I worked?	Yes, I have.	No, I haven't.
He's worked.	He hasn't worked.	Has he worked?	Yes, he has.	No, he hasn't.
She's worked.	She hasn't worked.	Has she worked?	Yes, she has.	No, she hasn't.
It's worked.	It hasn't worked.	Has it worked?	Yes, it has.	No, it hasn't.
You've worked.	You haven't worked.	Have you worked?	Yes you have.	No, you haven't.
We've worked.	We haven't worked.	Have we worked?	Yes we have.	No, we haven't.
They've worked.	They haven't worked.	Have they worked?	Yes they have.	No, they haven't.

The present perfect simple is used to discuss events that have just been completed at the moment of speaking.
For example:
Q) Have you done your homework?"
A) "Yes, I've just finished it."
It is often used to suggest that a past action still has an effect upon something happening in the present.
For example:
"The pound has fallen against the dollar."
It is also used to discuss unfinished time.
For example:
Q) Have you done your homework today?
A) No, I haven't done it yet.
Note - You are talking about today and today isn't finished, so you may do your homework later!
Q) Have you ever been to England?"
A) "Yes I have."
Note - You are talking about something that has happened in your life and your life isn't finished!
You can also use the present perfect to discuss something from the past but you don't want to say exactly when.
For example:
Q) "Are you learning any languages?"
A) "Yes, I've begun to learn English."
This tense is often used to discuss events that have been happening over a period of time, but aren't finished yet.
For example:
Q) "How long have you studied English for?"
A) "I've studied English for 2 years now."
However it is better (grammatically speaking) to use the Present Perfect Continuous to express yourself in this way.
For example:
Q) "How long have you been studying English for?" A) "I've been studying English for 2 years now."

Note ! It is always for a length of time and since a point in time.