since时间从句中,不论用短暂性动词、延续性动词还是状态动词的一般过去时,通常都表示从动作或状态的完成或结束时算起，因此这种since从句应理解为“自从该动作结束以来”，如:
Great changes have taken place here since he left. 自从他离开以来这里发生了巨大的变化。
▲从句中使用延续性动词时，常容易误解，要特别注意。如:
I haven't heard from him since he lived (或was) in Beijing.自从他离开北京,我没有收到过他的来信。
How long is it since you smoked?
你戒烟多久了(或你多长时间没抽烟了)?
▲如果要表示从句的动作或状态延续至今,该动词需用现在完成时，有时也可以用一般过去时，但这时需要在since前加ever,强调与现在的联系，如:
I've known her since I have lived here(或ever since I lived here /#since I came here).
自从我住到这儿以来我就认识她。
～～～～～～～～～～～～～～～～～～～
记住since的四种用法：
1.since +过去一个时间点。例如：
I have been here since 1992.
1992年起，我一直在这儿。
2. since +一段时间+ ago.例如：
We have been here since five months ago.
我们在这儿，已经有五个月了。
3. since +从句。例如：
Great changes have taken place in our hometown since you left. 你走后，咱们的家乡发生了巨大的变化。
4. It is +一段时间+ since从句。例如：
①lt is three years since the old man died.
那位老人去世已有3年了。
②It is five years since I joined the army.
我参军已有五年了。
练一练：用since和for的填空练习（牢记for后面接时间段而since后面接时间点。因为Since 用来说明动作起始时间，for用来说明动作延续时间长度。）
1. ______ two years
2._______ two years ago
3. _______ last month
4.______ 1999
5._______ yesterday
6. _______ 4 o’clock
7. ______ 4 hours
8._______ an hour ago
9. _______ we were children
10. _____ lunch time
11.______ she left here
12. He has lived in Nanjing ________ the year before last.
13. I’ve known him __________ we were children.
14. Our teacher has studied Japanese _______ three years.
15. She has been away from the city _________ about ten years.
16. It’s about ten years __________ she left the city.
答案：1、for 2、since 3、since 4、since 5、since 6、since 7、for 8、since 9、since 10、since 11、since 12、since 13、since 14、for 15、for 16、since
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