Ved å bruke presens perfektum (present perfect), kan vi definere en tidsperiode før nå, ved å ta i betraktning dens varighet, med for + en tidsperiode, eller ved å ta i betraktning dens utgangspunkt, med since + et tidspunkt. FOR og SINCE kan også brukes med preteritum perfektum (past perfect). SINCE kan bare brukes med perfekt tempus (perfect tenses). FOR kan også brukes med preteritum (simple past).
For + en tidsperiode
for six years, for a week, for a month, for hours, for two hours
I have worked here for five years.
Since + et tidspunkt
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.
Presens perfektum med 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.
Presens perfektum med 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.