a. when既可引导一个持续性动作,也可引导一个短暂性动作,可用于表示主句和从句动作同时发生或从句动作先于主句动作,有时还可表示从句动作后于主句。如：
     The film had been on when we arrived.当我们到达时,电影已经开演了。
     b. when在be about to do...when...,be doing...when...,had done...when...,be on one's way...when...,be on the point of doing...when...等结构中,作“就在那时候”讲,相当于(and then),表示某件事正在发生或刚刚发生时,另一动作同时发生。如：
     We were having a meeting when someone broke in.
     我们正在开会,突然有人闯了进来。
     I was about to go swimming when our guide stopped me.
     我正要去游泳这时向导把我拦住了。
     We were on the point of leaving when it suddenly began to rain.
     我们正要离开,忽然天开始下雨了。
     c. when还有“尽管、虽然；既然、鉴于；如果”等意思。
     He usually walks when he might ride.
     虽然他有车可坐,但他通常步行。(when引导让步状语从句,主要位于主句之后)
     They had only three tables when they needed five.
     他们虽然需要五张桌子,可是只有三张。
     How could I live in Beijing when all my favorite people live here?
     既然我最喜欢的人都在这里,我怎能住到北京去呢？
     How can you hope for mercy yourself when you show none?
     如果你不宽恕别人,你自己怎能希望得到别人的宽恕呢？