这个问题提得很好.这涉及到不定式的逻辑主语问题.
不定式并不像现在分词或过去分词那样表示明显的主动或被动关系,它的逻辑主语只能根据句子的意思去慢慢体会找出.不定式用主动结构表被动的形式总结如下:
1) sb have sth to do (不定式的动词所表动作由句子主语完成)
如: I have many flowers to water.(我给花浇水)
2) sb/sth be adj to do (介词).(实际上省略了for sb.不定式的逻辑主语是sb.)
如: English is diffilult to learn.
She is hard to work with.
3) There be sth to do(但也可使用to be done)（同样省略了for sb.不定式的逻辑主语是sb.）
如: There is still a lot of work to do.
4) with sb/sth to do sth(不定式的逻辑主语是后面句子的主语)
如: With a lot of work to do, I can't go out to play with you now.
再如: With you to help me, I can finish the work easily.
(当然该句只是用来和上句对比说明该动作并未发生,它并不是不定式主动表被动用法)
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