当不定式动词是不及物动词时,应加相应介词.
The room is too small for us to live in.
The ice is too thin to skate on.
the chair is very comfortable to sit on.
I have a firend to wait for.
这些不定式都是不及物动词,所以后面的介词不能省略.
The book is difficult to understand.
understand是及物动词,所以后面不跟介词.