保证准确!
1.When heated,metal expands.
2.If true,it will cause us a lot of trouble.
3.When in doubt,tell the truth.It will confound your enemies and astound your friends.
如果状语从句的主语是it或其和主句主语相同,且从句的谓语以be动词开头,则可以省略状语从句的主语和be动词,这就是所谓的非谓语从句.