不定式是英语动词的非限定形式之一，是英语中一种非常活跃的语言现象，不带to的不定式（bare infinitive）是我们学习、掌握和运用这一语法现象的难点。本文的目的就是要对不带to的不定式作一粗浅的探讨。首先要指出的是，不带to的不定式的使用实际上非常普遍，主要用在情态动词和助动词will，shall，would，should，can，could，may，might，must…以及在作为情态动词的need和dare之后。但因为这一点在学习英语的过程中极易掌握，故不在此赘述。除此以外，在下列情况下也要使用不带to的动词不定式。
1．在see，feel，hear，watch，observe，listen to，look at等感官动词或短语以及let，have，make等表示祈使意义的动词后面；不定式作宾语补足语不能带to；help后面的不定式可以不带to，也可以保留to；know在一定的条件下，后面的不定式作宾语补足语时可以不带to。例如：
i saw the man come ont of the house．
i heard her say that she was fed up．
we felt the house shake．
they watched the sun sink into the sea．
did you notice him pause？
they did not observe christine come in and go upstairs．
she sat listening to him climb the stairs．
look at the boy run！
i"ll let you know as soon as i hear from her．
i had him mend my watch．
electricity makes motors run．
he helped me（to） plant trees．
i have never known him sing so beautifully．
不带to的动词不定式跟在上述动词后面时，情况比较复杂，有以下几个问题须特别注意：
（1）当上述句子变成被动语态时，原来作宾语补足语的不定式则相应地成为主语补足语。这时，不定式就一定要带to，以上述句子为例：
the man was seen to come out of the house．
she was heard to say that she was fed up．
（后面的句子以此类推）其原因是“英语中还有一条规律：凡是被动的动词都不能和不带to的不定式连用。其中只有let除外，原因是作主语补足语的不定式是单音节词。例如：
the students were let go after class．
the grass was let grow．
（2）不定式在help之后，既可以用作宾语补足语，也可直接作宾语，例如：
this book will help（to）improve your english．
help之后不定式带to还是不带to可能有以下几种区别：①不带to结构在非正式文体中更为常见。②用不带to的不定式在美国英语中似乎比在英国英语中更常见。③决定不定式带不带to主要牵涉到主语，如果主语不能参与到不定式的动作中去，则不定式通常都要带to。例如：
this book helps to see the truth．
this book will help you to use english．
（3）have known后面跟的不定式可以不带to，也可以保留to。例如：
i have known her（to）lie．
never knew和have never known的后面通常跟不带to的不定式，但偶尔也可以见到带to的不定式。例如：
i never knew him do anything without a good reason．
i have never known him say this before．
i have never known her to tell lies．（张道真，1984）
（4）如果在feel，know，observe，see后面跟的不定式是be，则to不可省略。例如：
we all felt that to be the highest praise．
we know him to
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