19.8 比較while, when, as
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1)as, when 引導(dǎo)短暫性動(dòng)作的動(dòng)詞。
    Just as / Just when / When I stopped my car, a man came up to me.
2)當(dāng)從句的動(dòng)作發(fā)生于主句動(dòng)作之前,只能用when 引導(dǎo)這個(gè)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(shí)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。