When：当—时候
1. 当从句的动作发生在主句之前，只能用when, 不能用as/while，
2. when 引导的动作既可以是持续的，也可以是短暂的。
3. 遵循主将从现原则（主句是将来时，从句是一般现在时）
When she comes, I will tell her the bad news 当她来的时候，我将在这里告诉这个坏消息
I loved history when I was at school. 我上学时喜欢历史
Call me when you've finished. 你完成后就打电话给我
真题举例：The ethical judgments of the Supreme Court justices have become an important issue recently. The court cannot maintain its legitimacy as guardian of the rule of law A justices behave like politicians. Yet, in several instances, justices acted in ways that weakened the court’s reputation for being independent and impartial. (2012 完型) 最近，最高法院法官的道德判断已经成为一个重要问题。当法官的行为举止像政客，法院无法维持其作为法治守护者的合法性。然而，在一些案例中，法官的行为方式削弱了法院独立和公正的声誉。
[A] when [B] lest [C] before [D] unless
while 在---期间，在---过程中，在—同时，在---时候
while和as/when比起来，多引导持续性动作, 不能引导终止性动词（got to, finish等）
Strike while the iron is hot 趁热打铁
While my wife was reading the newspaper, I was watching TV(同时发生)
真题例句：While producing large quantities of CO2, these computers emit a great deal of heat, so the centers need to be well air-conditioned, which uses even more energy. （2011 翻译）在产生大量二氧化碳的同时，这些计算机也会散发出大量的热量，因此这些中心需要良好的空调环境，而这就会消耗了更多的能源
（3）as，随着，边—边 ，当---时候
表示时间推移，只能用as，强调主从动作同时发生，引导的动作多是延续性的
we always watch TV as we have dinner 我们一边吃饭一边看电视
The students took notes as they listened 学生们边听课边记笔记
As she grew older she gained in confidence. 随着年龄的增长她的信心增强了
真题例句：As the brain fades, we refer to these occurrences as “senior moments.”型) 当大脑衰退时，我们把这种情况称为“老年时刻”。(2014 真题）
注意：三者均可表示当—时候，如果主句表示短暂性动作，三者都可用。
I met Li Kang when/while/as I was walking along the road我在路上走的时候遇到了李康