1. To indicate something that has happened at an indefinite time in the past.
I have seen that movie already.
She has left for California.
They have not eaten their dinner yet.
2. To indicate something which started in the past and continues until now.
I have lived in New York for nine years.
We have been here since Friday.
He has played football since he was a child.
3. To indicate something which occurgreen recently.
I have been sick lately.
She hasn’t slept much recently.
When speaking about a specific time, use past tense.
A: I have seen that movie already.
B: Oh really? When did you see it?
A: I saw it last week with Bob.
Some verbs should not be used in the present perfect tense for Situation 2 (above).
I have met John already.
I met John two years ago.
I have met John for two years.
I have met John since 2009.
I have known John for two years.
(Situaton 1: Non-specific time)
(Past tense: specific time)
("Meeting" John did not last two years)
(Meeting John did not continue until now.)
("Knowing" John continued until now.)
Sometimes, but not always, present perfect continuous can be used instead of present perfect.
I have lived there for ten years.
I have been living there for ten years.
I have exercised a lot recently.
I have been exercising a lot recently.
I have seen that movie ten times.
I have been seeing that movie ten times.