when等于：what time
The film was already on when we arrived .我们到时，电影已经开演了。
When two tigers fight ， one is sure to lose ...两虎相争，必有一伤。
When are you coming over to see us again 你什么时候再来看我们呢
Plastics will soften when exposed to heat .塑料适当加热就可以软化。
When is he dejected or melancholy ?什么时候他才会感到失意或悲伤呢
When a car is out of gear it cannot run .汽车脱档后，就不能开动了。
