when可当疑问副词,也可当关系副词.1.when当疑问副词,一定是放在句首.例:When is the play going to start?这场戏什麼时候要开始?2.when当关系副词,用来修饰表时间的先行词,可用来代替"时间介系词+which",如" at whic...