當我們在使用現在完成式時，如果考量的是動詞的持續時間，就以 for + 一段時間來定義持續時間的長短；如果考量的是動作開始的時間點，就用 since + 一個時間點即可。「for」和「since」也可以用於過去完成式。「since」只能用在完成式，而「for」除了用在完成式，也可以用於過去簡單式。
for + 一段時間
for six years, for a week, for a month, for hours, for two hours
I have worked here for five years.
since + 時間點
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.