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.