在现在完成时用「for」和「since」
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.
for 用於现在完成时
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.
since 用於一般现在时
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.