a chair to sit on;a pen to write with.这种怎么判断最后要不要加介词?从句中，比如说：Although the lecture had already been on for five minutes ,I still was not able to find a chair_________.这是to sit 还是to sit on .为什么？
问题描述：
a chair to sit on;a pen to write with.这种怎么判断最后要不要加介词?
从句中，比如说：Although the lecture had already been on for five minutes ,I still was not able to find a chair_________.
这是to sit 还是to sit on .为什么？
答
你就看正常语序要不要加介词，比如第一个必须是sit on the chair，所以修饰chair的时候就要用on，第二个是write with a pen，所以后面要带with
就是sit on 啊，因为必须是sit on a chair 而不是sit a chair
答
write with a pen
答
这种东西+to+动词的就倒过来，to sit on a chair，所以有on
答
这是倒装句，等于 sit on a chair, write with a pen.
倒过来看你就知道是否需要加介词了。
答
这是一种后置定语,是用动词不定式做定语,你可以把to sit on 带入前句的a chair里
sit是不及物动词,后边想加宾语要用介词连接,“sit on a chair”是固定搭配,所以要加介词on