Através do uso do "present perfect", podemos definir um período de tempo anterior ao momento presente, considerando sua duração, caso no qual utilizamos for + período de tempo, ou considerando seu ponto de partida, em que empregamos since + momento concreto. "For" e "since" podem ainda ser utilizados no "past perfect". "Since" admite apenas tempos verbais perfeitos. "For" pode também ser utilizado no "simple past".
"For" + período de tempo
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" + "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" + "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.