1. 區分方式
Since + point in time 特定時間點
For + length of time 時間長度
2. 例句
(1) I have worked here since 1990. 我從西元1990年開始就在這裡工作了。
I have worked here for 20 years. 我已經在這裡工作二十年了。
(2) We have lived in Mexico since 2005. 我們從西元2005年開始就住在墨西哥。
We have lived in Mexico for 5 years. 我們已經住在墨西哥五年了。
3. John的例子
(1) John has been a student since 2000 / for ten years.
John從2000年起就是個學生。/John已經當了學生十年了。
(2) John has lived in Canada since 2005 / for 5 years.
John從2005年起就住在加拿大。/John已經住在加拿大五年了。
(3) John has been married since 2008 / for 2 years.