Il present perfect definisce un periodo di tempo antecedente al presente in considerazione della sua durata, con for + un periodo di tempo, o in considerazione del punto di partenza, con since + un momento preciso. For e since possono essere usati anche con il past perfect. Since può essere usato esclusivamente con i tempi perfect (present perfect e past perfect) mentre for può essere usato anche con il simple past.
For + periodo di tempo
for six years, for a week, for a month, for hours, for two hours
I have worked here for five years.
Since + momento preciso
since this morning, since last week, since yesterday
since I was a child, since Wednesday, since 2 o'clock
I have worked here since 1990.
Present perfect con "for"
She has lived here for twenty years.
We have taught at this school for a long time.
Alice has been married forthree months.
They have been at the hotel for a week.
Present perfect con "since"
She has lived here since 1980.
We have taught at this school since 1965.
Alice has been married since March 2nd.
They have been at the hotel since last Tuesday.