B7U2语法：不定式的被动语态（to be done）用法补充
❀不定式有时态和语态的变化，时态共四种，即一般式、完成式、进行式和完成进行式。
1 现在时：一般现在时表示的动词，有时与谓语动词表示的动作同时发生，有时发生在谓语动词的动作之后。
He seems to know this. I hope to see you again.= I hope that I’ll see you again.
2 完成时：表示的动作发生在谓语动词表示的动作之前。
I’m sorry to have given you so much trouble. He seems to have caught a cold.
3 进行时：强调动作已在延续或动作正在进行。
They seem to be getting along well with their new jobs. He seems to be eating something.
4 完成进行时：表示动作是在谓语动词所表示的动作之前就一直在进行。
They are said to have been collecting stamps for over twenty years.
He appears to have been waiting a long time.
❀不定式的主动形式表示被动
1. 在there be结构中。例如：
There is a lot of homework to do．(也可用to be done) There is no time to lose (to be lost). 2. 在“n／pron + be + adj + to do”结构中。常用的形容词有easy，difficult，hard，impossible，nice，pleasant，light，heavy，interesting，important，expensive，cheap，fit，dangerous 等。例如：
He is hard to convince. He is an impossible person to work with．
3. 在“too…to do; enough…to…”结构中。如：
The problem is too difficult to work out (to be worked out)．The house is big enough to live in.
4. 在“with+n+to do”结构中。例如：
With nothing to do, he lay in bed．With so many exercises to do, I can’t go to the cinema．
5. 当不定式的逻辑主语在句中出现时。例如：
I have a letter to type today．(I是to type的逻辑主语)
Do you have anything to say? (you是to say的逻辑主语)
6. 当不定式隐含在for sb to do结构中时。
This is the best book to read (=for us／you to read)．
The important thing to do is to lock the door when we leave the house．
7. 一些作表语用的不定式的主动形式。常见的这类动词有let(出租), rent，hire, blame等。
The house is to let．I felt I was to blame.
❤注意！！动词不定式主动语态形式的使用要比被动语态形式的使用普遍，有时（尤其在口语中）虽然意思上是被动的，也往往用主动语态形式来表示。
I have three motors to repair today. The task is difficult to finish on time.
