Present perfect tense

Present Perfect Tense
For & Since
We often use for and since with the present perfect tense.
* We use for to talk about a period of time - 5 minutes, 2 weeks, 6years.
* We use since to talk about a point in past time - 9 o'clock, 1st January, Monday.
for since
a period of time a point in past time
20 minutes 6.15pmthree days Monday
6 months January
4 years 1994
2 centuries 1800
a long time I left school
ever the beginning of time

Here are someexamples:
* I have been here for 20 minutes.
* I have been here since 9 o'clock.
* John hasn't called for 6 months.
* John hasn't called since February.
*He has worked in New York for a long time.
* He has worked in New York since he left school.
***For can be used with all tenses. Since is usually used with perfect tensesonly.
Just, already & yet
You can use the present perfect with just = (short time ago)
Examples
I have just lost my umbrella.
Ow! I have just burntmyself.
Oh...I have just forgotten her address.
You can use the present perfect with already: when something has happened sooner than expected.
Examples
Don'tforget to post the letter ‘I have already posted it'.
Have you washed your hair? 'I have already washed it'
When is he going to start his job? 'He has already started it'.You can also use the present perfect with yet: Yet is used only in questions and negative sentences. It shows that the speaker is expecting something to happen.
ExamplesI haven't told her about the accident yet.
I haven't seen that movie yet...But I am going to soon.
Have you been to Vienna yet?
Has it stopped raining yet?