动词不定式的定义
(语态)动词和参与此动作的主语之间关系的一个术语。当主语是动作的发起者(或之一)时，称为主动语态;
如果动词不定式的逻辑主语是这个不定式所表示的动作的承受者，不定式一般要用被动语态形式。如：
It's a great honour to be invited to Mary's birthday party.(不定式作主语是被动语态to be invited是被邀请)
It was impossible for lost time to be made up.(不定式作主语)
I wish to be sent to work in the country.(不定式作宾语)
Can you tell me which is the car to be repaired?(不定式作定语)
He went to the hospital to be examined.(不定式作状语)
在There be结构中，修饰主语的不定式可用被动，也可用主动。如：There are still many things to take care of (to be taken care of).但有时两种形式表达的意思不同，如：There is nothing to do now.( We have nothing to do now.) There is nothing to be done now.(We can do nothing now.)
形式
1) 现在式：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. 我希望再见到你。
2) 完成式：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)进行式：表示动作正在进行，与谓语动词表示的动作同时发生。
He seems to be eating something.
4)完成进行式：
She is known to have been wreaking on the problem for many years.
一般在表示情绪的动词后加to do也表将来
疑问词
疑问词who,what,which,when,where,whether,how后可接不定式构成不定式短语，在句中作主语、宾语、表 语等。如：
①When to leave for London has not been decided yet. (不定式在句子中做主语)
②Mr. Smith didn't know whether to leave or stay there. (不定式在句子中做宾语)
③I asked Professor Xu how to learn English well. (不定式在句子中做直接宾语)
④The question was where to get the medicine needed. (不定式在句子中表语)
以上例句中疑问词+不定式部分，均可转换为相应的从句形式。如：①When we shall leave…③…how I
could learn……
经常在这种结构中使用的动词有：consider,decide,discover,explain,find out,forget,hear,know,lea rn,observe,understand,wonder等。
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