表示对比,主要有以下两种情况：
A. 相当于 whereas, while, since, 意为“既然；然而”.
如： （ 1 ） How can I help them to understand when they won't listen to me? 既然他们不愿听我的,我又怎么能帮助他们弄明白呢?
（ 2 ） They have only three copies when we need five. 他们只有三本,而我们却需要五本.
B. when 从句中使用虚拟语气,意为“本来……却……”.
如： （ 1 ） She paid when she could have entered free. 本来可以免费入场,而她却付了款.
（ 2 ） She stopped trying when she might have succeeded next time. 她本来下次有可能获得成功,但她却不再尝试了.
（ 3 ） They were gossiping, when they should have been working. 他们本该工作,却在那里说别人的闲话.