在使用现在完成时的时候，如果考量的是动词的持续时间，就以 for + 一段时间来定义持续时间的长短；如果考量的是动作开始的时间点，就用 since + 一个时间点。「for」和「since」也可以用於过去完成时。「since」只能用在完成式，而「for」除了用在完成时，也可以用於一般过去时。
for + 一段时间
for six years, for a week, for a month, for hours, for two hours
I have worked here for five years.
since + 时间点
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.
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.
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.