英語四級語法必備：比較while，when，as

　　1)as, when 引導(dǎo)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(fā)生于主句動作之前，只能用when 引導(dǎo)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。