1.When 现在分词.例如:
When sleeping,I never hear a thing.睡觉时,我什么都听不见.
2.When 过去分词.例如:
This dictionary will look nice when printed.这本词典印出来时会很好看.
3.When 名词或形容词.例如:
When a boy,he was sick all the time.小的时候,他经常生病.
4.When 介词短语.例如:
When at Rome do as the Romans do.入乡随俗.
