当你考试时英语可以用 "take an exam" 或 "sit for an exam" 来表示。

这两个短语都是来自英语中常用的表达方式，在英语中，"take" 常用来表示进行某种活动或进入某种状态，例如：

I'm going to take a nap. (我要睡觉。)
She took a shower before going to bed. (她睡觉前洗了个澡。)

而 "sit" 在这里的意思是坐在某个地方或者某个位置，例如：

He sat down on the couch. (他坐在沙发上。)
She sat at the table and started to eat. (她坐在桌子旁边开始吃饭。)

所以，当你考试时英语可以用 "take an exam" 或 "sit for an exam" 这两个短语来表示。

一些例句：

I have to take an exam tomorrow. (我明天要参加考试。)
She's going to sit for an exam next week. (她下周要参加考试。)
He took an exam yesterday. (他昨天参加了考试。)
They're sitting for an exam today. (他们今天在参加考试。)