是时间状语从句，例如：
1、The house is a little more dilapidated than when I last saw it那房子比我上次见到时更破旧了些。
2、You invariably leave your health club a darned sight less healthy than when you went in.人们离开健身俱乐部的时候，气色看上去总是比刚进去时要糟糕。
3、Mustard is grown in the field when weeds are there， rather than when the growing crops are there.芥菜要在地里长杂草而不是长庄稼时种植。
