| ago 以前，从前 | since 自…以后，自…以来 | for 在…时，持续了... |
|---|---|---|
| 通常搭配一般过去时 | since + 过去的时间 = 现在完成时 | 无时态要求(可搭配任意) |
| They won the match three days ago. | Since 1992, we have lived in this city. | They have been practicing for a year. |
- He stayed with us for two weeks.
- She arrived two weeks ago.
- I have not seen him since Monday.
- He left a month ago and I have not seen him since then.
- We have been working on this new plane for over a year now.