介词后面是加主语的宾格，应在主动变被动中，在主动变被动中，主语变成by的宾语，原先是主格代词要变成宾格代词。
介词的基本用法:
1、后跟名词:atschool在上学，inperson亲自， on duty值班，atwork在上班，byhand用手， with pleasure乐意等。
2、后跟代词:I'm on it 我来办，stand byhim站在他身边，few ofthem他们中的少数几个，part ofit其中一部分，depend on you指望你。
3、后跟动名词:do wellin dancing擅长跳舞，be good at swimming 擅 游泳，how about going traveling 去旅行怎么样 爱骑
行。
介词短语在句子中的语法功能是:
1.定语.如:The key to the door is missing.
2.状语.如:They are playing basketball on the playground.
3.表语.如:Japan is to the east of China.
4.宾语补足语.如:We consider education of great importance.
5.主语补足语.如:These machines must be kept in good cindition
介词后面不跟主语，跟动词或者代词。
如 A cup is on the desk.
