before，after, since, when引导的时间从句可以修饰名词，这在本网已经有过多次讨论了。虽然语法书在讲从句的时候，没有专门讲述这种从句，但现实语言中的确存在时间连词引导的从句作名词修饰语的现象。

看起来when为整个句子的状语，但是我说过我们要分析深层次的东西，外在可以多样，深层一定。补充为imagine that we were surpried when he walked into the room.继续完善，imagine that we were surpried the moment when he walked into the room.可见实质作定语。

Imagine our surprise when he walked into the room!
when引导的时间从句可以修饰名词。如果理解不了或无法解释，还有一种曲折办法。把 our surprise 这个名词词组变成一个宾语从句，让when引导的时间从句修饰这个宾语从句。从理解句意和结构的角度，可以这样处理：

Imagine how surprised we were when he walked into the room!