714　when，while，after，before，if+分词等

① when，while引导状语从句时，如果从句的主语同主句的主语为同一人或物，从句中的主语及助动词be常可省略，采用“从属连词+现在分词、过去分词或介词短语等”结构。

He met an old friend when walking along the road. 在路上走着时，他遇见了一位老朋友。（=When he was walking along the road）

Be careful when crossing the road. 过马路时要当心。

When reading the letter，she couldn't help crying. 阅读这封信时，她不禁哭了起来。（=When she was reading the letter）

While talking in the room，they heard a strange sound outside. 在房间里说话时，他们听见外面有一个奇怪的声音。（=While they were talking in the room）

While doing so，he decided to buy a car. 他做这事时决定买一辆车。（=While he was doing so）

When in trouble，ask her for help. 遇到麻烦时就请她帮忙。（=When you are in trouble）

While in London he studied English very hard. 他在伦敦时努力学习英语。（=While he was in London）

② after和before引导时间状语从句时，如果从句的主语同主句的主语为同一人或物，常可转换为“after、 before+现在分词”结构。

After going over the lessons，she went to bed. 她复习完功课后就睡了。（=After she went over the lessons）

After having worked for a whole day，he felt a bit tired. 工作了一整天之后，他感到有点累。

Before moving to Shanghai，she had lived in Hangzhou for twenty years. 在搬到上海之前，她已在杭州住了20年。（=Before she moved to Shanghai）

Before beginning to do his homework，he had watched TV half an hour. 在开始做作业之前，他已看了半个小时的电视。（=Before he began to do his homework）

③ if引导的从句如果是被动语态，而且从句的主语同主句的主语又相一致时，从句中的主语和be动词可省略，转换为“if+过去分词”结构。

If given more time，I could do the work better. 如果给更多时间的话，这项工作我可以做得更好。

If given another chance，he may succeed. 如果再给一次机会的话，他可能会成功的。