一、时间状语从句
as/when/while
1._________I was walking down the street，I noticed a police car in front of No.37.
When/while/As=During the time that (时间段)
2._________ Mary was writing a letter, the children were playing outside.
While=At the same time as 与……同时
3.The volume varies _________ the temperature increases.
as 体积随着温度增加而变化。
4.________ he stood up，he dropped the glass.
As/When=At the time that (时间点)
练习：
1. ____________I had done it，I knew I had made a mistake.
A. Direct B. Directly
C. Moment D. Constant
二、条件状语从句
if 好的条件，可用 provided/providing/suppose/supposing(that)/ on condition that 替换
不好的条件可用 so/as long as替换
如：If/Provided(that)it snows tomorrow, we shall play snowballs.
If/So long as it snows tomorrow, we shall delay our trip to the seaside.
练习：
1. We’ll visit Europe next year ______ we have enough money.
A. lest B. until C. unless D. provided
only if/if only
1.only if=only on condition that“只要，只有…”
如：Only if a teacher has given permission is a student allowed to enter this room.(主句例装)
只有得到教师的允许，学生才可以进这间屋。
2.If only“要是…就好了”(虚拟)
如：If only we had arrived in time，we would not have missed the train.
if/unless
unless=if…not (且语气较强)
如：You will make progress if you study hard.
比较：You will never make progress unless you study hard.
Unless he comes，we won’t be able to go.