请注意以下三个句式的区别和意思上的差异（其中的 is 根据情况可以有其他时态形式）：
It is + 时间段 + since… 的意思是“自从……以来有一段时间了”。
It is + 时间点 + when… 的意思是“当……的时候时间是……”。
It is + 时间段 + before… 的意思是“在……之前有一段时间”。
请看以下用例：
It’s ten years since they got married. 他们结婚到现在已经十年了。
It was past one o’clock when he came in. 他回来时已经一点多了。
It won’t be long before they understand each other. 过不了多久他们就会互相了解的。
有时翻译可以比较灵活：
It’s a long time since I met you last. 好久不见了。(直译：自上次见到你以来有很长一段时间了。)
It’s about three years since I last played basketball. 我差不多有三年没有打篮球了。(直译：自从我上次打篮球以来已差不多有三年了。)
请看有关高考题(答案为DDBCDC)：
1. That was really a splendid evening. It’s years _________ I enjoyed myself so much. (安徽卷)
A. when B. that C. before D. since
2. It was some time _________we realized the truth. (山东卷)
A. when B. until C. since D. before
3. ―Did Jack come back early last night?
―Yes. It was not yet eight o’clock _________he arrived home. (福建卷)
A. before B. when C. that D. until
4. Scientists say it may be five or six years _________it is possible to test this medicine on human patients. (福建卷)
A. since B. after C. before D. when
5. It was evening we reached the little town of
A. that B. until C. since D. before
6. It _________ long before we _________ the result of the experiment. (上海春)
A. will not be; will know B. is; will know
C. will not be; know D. is; know
【全真模拟训练】
1. It was about midnight _________ we found the lost child, crying under a tree.
A. that B. until C. when D. where
2. It's some time _________ I have spoken to you about this matter.
A. that B. unless C. since D. until
3. It was ten _________ he stood at aunt’s door and rang the bell.
A. when B. until C. since D. that
4. I think it won’t be long _________ they understand each other.
A. when B. until C. since D. before
5. How pleased I am tonight! It’s years _________ we had so great a time together.
A. when B. that C. before D. since
6. It was some time _________ we realized how important the matter is.
A. when B. until C. since D. before
7. ―Did you come home very late last night?
―No. It was just half past seven _________ I appeared before my parents.
A. that B. when C. until D. since
8. How time files! It seems like years _________ I last saw you.
A. when B. that C. since D. before
9. It’s more than half a century _________ the People’s Republic of China was liberated.
A. when B. that C. since D. after
答案：1―5 CCADD 6―9 DBCC