全国大众英语测验语法——比较while,when,as
1）as, when指导长久性举措的动词。
Just as / Just when / When I stopped my car, a man came up to me.
2）当从句的举措发作于主句举措之前，只能用when指导这个从句，不可用as或 while。
When you have finished your work, you may have a rest.
3）从句默示"随时候推移"连词能用as，不用when或while。
As the day went on, the weather got worse.
日子一每天过往，气候越变越坏。

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