按结构
单句
只有一个主部（主语）加一个述部【包括动词，补语，受词（宾语）】，称为单句。 ①I have a sufficient income to support you.我有足够的收入养你。 ②Happy memories came and went in my mind.快乐的回忆在我的脑海里流过。 ③Jack and Jason rushed into the classroom.杰森和杰克冲进了教室。
合句
由对等连接词连接的两对等子句，成为合句。 ①I missed the last train,so i took a taxi home.我错过了末班车，所以搭计程车回家。（介系词+名词）②The driver tried to control the car,but it crashed/bump into a tree.司机试着控制那辆车，但他还是撞上了树干。air craft航空器；air crash空难；bumper保险杠
复句
由从属连接词所连接的主句及从属子句。①Who did it is still a mystery.谁做了这件事仍是个谜。 ②He is the man who saved my son from drowing.他就是从水中救起我儿子的那个人。 ③I want to finish the work before it gets dark.我想在天黑前完成这个工作。 =Before it gets dark,i want to finish work.
复合句
用对等连接词将单句与复句或复句与复句结合成句。 ①She knew what she wants,but she never told anyone.他知道自己想要什么，但她从不告诉别人。 ②I know the girl who is talking with Mr.Green but my brother doesn't know who she is.我认识那位正在和格林先生聊天的女孩，但我哥哥不知道她是谁。第一个who是关系代名词，第二个who是疑问词。关系代名词不需要翻译；疑问词需要翻译。
