在現在完成式使用「for」和「since」
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.
for 用於現在完成式
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 用於現在簡單式
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.