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。來源：www.examda.com
     When you have finished your work, you may have a rest.來源：www.examda.com
    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