Empleando el "present perfect" podemos definir un periodo de tiempo anterior al momento presente, considerando bien su duración, caso en el que utilizamos "for" + periodo temporal,, o bien su inicio o punto de partida, caso en el que utilizamos "since" + momento concreto. "For" y "since" pueden asimismo emplearse con el "past perfect". "Since" admite únicamente tiempos verbales perfectos. "For" puede también emplearse con el "simple past".
"For" + periodo de tiempo
for six years, for a week, for a month, for hours, for two hours
I have worked here for five years.
"Since" + momento concreto
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.