Using the present perfect, we can define a period of time before now by considering its duration, with for + a period of time, or by considering its starting point, with since + a point in time.
For + a period of time
* for six years, for a week, for a month, for hours, for two hours.
* I have worked here for five years.
Since + a point in time
* 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 with for
* She has lived here for twenty years.
* We have taught at this school for a long time.
* Alice has been married for three months.
* They have been at the hotel for a week.
present perfect with 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.
Note:
* For and since can both be used with the past perfect.
* Since can only be used with perfect tenses, for can also be used with the simple past.
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