带to不定式有逻辑宾语在前时，to后动词用及物动词；不及物时需跟介词。The work is too hard for me to do. (work做及物动词do的逻辑宾语)

　　I have nothing to say. (nothing做及物动词say的逻辑宾语)

　　I have a letter to write. (letter是及物动词write的逻辑宾语) 而以下情况有介词：

　　I have a pen to write a letter with. ( 用钢笔写信，所以要有“用”字，故加with.)

　　The zoo is not a good place for animals to live in.

　　(“住在动物园里”，zoo是live in的逻辑宾语，故不及物动词live加介词in. )

　　Tom has no friends to talk to. (friends 是talk to的逻辑宾语，to不可少。)

　　He runs too fast for us to catch up with. (he是catch up with的逻辑宾语，带with.)

　　There are no seats to sit on. (坐在座位上，故sit后要加on.)

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