Ved at bruge førnutid kan vi definere en tidsperiode før nu ved at tage dens varighed i betragtning med for + en tidsperiode eller ved at tage dens udgangspunkt i betragtning med since + et tidspunkt. FOR og SINCE kan også begge bruges med førdatid. SINCE kan kun bruges med de fuldendte tider (perfect tenses). FOR kan også bruges med præteritum (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.
Førnutid 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.
Førnutid 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.