小學英語語法知識點：比較while, when, as

　　1）as， when 引導短暫性動作的動詞。

　　Just as / Just when / When I stopped my car， a man came up to me.

　　2）當從句的動作發生于主句動作之前，只能用when 引導這個從句，不可用as 或 while。

　　When you have finished your work， you may have a rest.

　　3）從句表示“隨時間推移”連詞能用as，不用when 或while。

　　As the day went on， the weather got worse.

　　日子一天天過去，天氣越變越壞。