3. 不定式作主语

　　1) It's easy (for me) to do that. 我做这事太容易了。

　　easy, difficult, hard, important, possible, impossible, comfortable, necessary, better;

　　the first, the next, the last, the best, too much, too little, not enough

　　It's so nice to hear your voice.　听到你的声音真高兴。

　　It's necessary for you to lock the car when you do not use it. 当你不用车的时候，锁车是有必要的。

　　2) It's very kind of you to help us. 他帮助我们，他真好。

　　kind, nice, stupid, rude, clever, foolish, thoughtful, thoughtless, brave, considerate(考虑周到的), silly, selfish(自私的)

　　It was silly of us to believe him.　我们真愚蠢，竟然相信了他。

　　It seemed selfish of him not to give them anything.　他不给他们任何东西，这显得太自私了。

　　注意

　　1) 其他系动词如，look，appear等也可用于此句型

　　2) 不定式作为句子成分时，动词用单数形式。

　　3) 当不定式作主语的句子中又有一个不定式作表语时，不能用It is… to…的句型

　　(对)To see is to believe.　百闻不如一见。(错)It is to believe to see.

　　It's for sb.和 It's of sb.

　　1) for sb. 常用于表示事物的特征特点，表示客观形式的形容词，如easy, hard, difficult,　 interesting, impossible等：

　　It's very hard for him to study two languages. 对他来说学两门外语是很难的。

　　2) of sb的句型一般用表示人物的性格，品德，表示主观感情或态度的形容词，如good, kind, nice, clever, foolish, right。

　　It's very nice of you to help me. 你来帮助我，你真是太好了。

　　for 与of 的辨别方法

　　用介词后面的代词作主语，用介词前边的形容词作表语，造个句子。如果道理上通顺用of，不通则用for。如：You are nice.　(通顺，所以应用of)。He is hard.　(人是困难的，不通，因此应用for。)

　　4. 不定式作表语

　　不定式可放在be动词后面，形成表语。例如：

　　My work is to clean the room every day.

　　His dream is to be a doctor.

　　5. 不定式作定语

　　不定式做定语通常要放在被修饰的词后。例如：

　　I have a lot of work to do.

　　So he made some candles to give light.

　　6. 不定式作状语

　　1) 目的状语

　　To… only to (仅仅为了), in order to, so as to, so(such)… as to… (如此……以便……)

　　He ran so fast as to catch the first bus.　他飞快地跑以便赶上第一班车。

　　I come here only to say good-bye to you.　我来仅仅是向你告别。

　　2) 作结果状语，表事先没有预料到的，要放在句子后面。

　　What have I said to make you angry.

　　He searched the room only to find nothing.

　　3) 表原因

　　I'm glad to see you.

　　典型例题

　　The chair looks rather hard, but in fact it is very comfortable to ___.

　　A. sit　 B. sit on　 C. be seat　 D. be sat on

　　答案：B.　如果不定式为不及物动词，其后应有必要的介词。当动词与介词连用时，常位于"形容词+动词不定式"结构的末尾。

　　用作介词的to

　　to 有两种用法：一为不定式+动词原形; 一为介词+名词/动名词, to 在下面的用法中是第二种，即to+ 名词/动名词：admit to承认，confess to承认，be accustomed to 习惯于，be used to 习惯于，stick to 坚持，turn to开始，着手于，devote oneself to 献身于，be devoted to 致力于， look forward to 盼望，pay attention to

　　注意

　　省to 的动词不定式

　　1) 情态动词 ( 除ought 外，ought to)：

　　2) 使役动词 let, have, make：

　　3) 感官动词 see, watch, look at, notice , observe, hear, listen to, smell, feel, find 等后作宾补，省略to。

　　注意

　　在被动语态中则to 不能省掉。

　　I saw him dance. =He was seen to dance.

　　The boss made them work the whole night.=They were made to work the whole night.

　　4) would rather，had better：

　　5) Why… / why not…：

　　6) help 可带to，也可不带to, help sb (to) do sth：

　　7) but和except：but前是动词do时，后面出现的动词用不带to的动词不定式。

　　8) 由and, or和than连接的两个不定式，第二个to 可以省去：

　　9) 通常在discover, imagine, suppose, think, understand等词后，可以省去to be：He is supposed (to be) nice.　他应该是个好人。举例：He wants to move to France and marry the girl. He wants to do nothing but go out. 比较：He wants to do nothing but go out. He wants to believe anything but to take the medicine.