I don't understand why Catherine stopped short when she ought to have continued. It's obvious that she lacked self-confidence.

【翻译】我不明白凯瑟琳为什么停下来了，（而此刻）她本该继续下去的。很明显，她缺乏自信。

when (用于引出使句子的其他部分显得让人吃惊或不大可能的事实或评论) 然而,可是

You use when in order to introduce a fact or comment which makes the other part of the sentence rather surprising or unlikely.

Our mothers sat us down to read and paint, when all we really wanted to do was to make a mess... 

我们的母亲让我们坐下来读书和画画，而我们真正想做的却是打打闹闹。

The temperature sensor is making the computer think the engine is cold when, in fact, it's hot. 

温度传感器发给计算机的信息为发动机是冷却的，可实际上它的温度很高。