The Present Perfect is a verbal tense if you use the auxiliar verb Have/Has (to have), the past participle of the verbs (Been, Done, Had, etc...) and the adverbs yet, already, just still etc...
It is use when the action happens between the past and the present,Well, you use this tense to say an action in present but doesn't finish:
Example:
Positive: You have already seen that movie
Negative: He hasn't seen that movie yet.
Interrogative: have they seen that movie yet?
PRESENT PERFECT + YET, ALREADY:
We use already in affirmative sentences,
We use yet in interrogative and negative sentences.
Ex:
Have you done the exercices yet?
Yes, I have already done it / No, I haven't done it yet.
PRESENT PERFECT + JUST, STILL:
To express an action that finished very recently, we put just before the principal verb:
I've just arrived. He has just phoned.
Have they just bought that car?
To express an action that is continuing, we use still in affirmative, negative and interrogative sentences:
Are you still in the bath? Yes, I'm still having a shower / I still haven't done my hair.
PRESENT PERFECT + SINCE & FOR:
When we talk about Since and For we are talking of the period of tense contable or a concret date...
Ex:
-We've lived in this town since 1990
-She's been in bed for twelve hours!
BIBLIOGRAPHY:
The book of grammar 4 2nd E.S.O