It's for sb。和 It's of sb。
1) for sb. 常用于表示事物的特征特點，表示客觀形式的形容詞，如easy, hard, difficult, interesting, impossible等：
It's very hard for him to study two languages. 對他來說學(xué)兩門外語是很難的。
2) of sb的句型一般用表示人物的性格，品德，表示主觀感情或態(tài)度的形容詞，如good, kind, nice, clever, foolish, right。
It's very nice of you to help me. 你來幫助我，你真是太好了。
for 與of 的辨別方法
用介詞后面的代詞作主語，用介詞前邊的形容詞作表語，造個句子。如果道理上通順用of，不通則用for。如：You are nice. (通順，所以應(yīng)用of)。He is hard. (人是困難的，不通，因此應(yīng)用for。)
4. 不定式作表語
不定式可放在be動詞后面，形成表語。例如：
My work is to clean the room every day。
His dream is to be a doctor。
5. 不定式作定語
不定式做定語通常要放在被修飾的詞后。例如：
I have a lot of work to do.
So he made some candles to give light。
6. 不定式作狀語
1) 目的狀語
To… only to (僅僅為了)， in order to, so as to, so(such)… as to… (如此……以便……)
He ran so fast as to catch the first bus. 他飛快地跑以便趕上第一班車。
I come here only to say good-bye to you. 我來僅僅是向你告別。
2) 作結(jié)果狀語，表事先沒有預(yù)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。
典型例題
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. 如果不定式為不及物動詞，其后應(yīng)有必要的介詞。當(dāng)動詞與介詞連用時，常位于"形容詞+動詞不定式"結(jié)構(gòu)的末尾。
用作介詞的to
to 有兩種用法：一為不定式+動詞原形; 一為介詞+名詞/動名詞， to 在下面的用法中是第二種，即to+ 名詞/動名詞：admit to承認，confess to承認，be accustomed to 習(xí)慣于，be used to 習(xí)慣于，stick to 堅持，turn to開始，著手于，devote oneself to 獻身于，be devoted to 致力于， look forward to 盼望，pay attention to
注意
省to 的動詞不定式
1) 情態(tài)動詞 ( 除ought 外，ought to)：
2) 使役動詞 let, have, make：
3) 感官動詞 see, watch, look at, notice , observe, hear, listen to, smell, feel, find 等后作賓補，省略to。
注意
在被動語態(tài)中則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 可帶to，也可不帶to, help sb (to) do sth：
7) but和except：but前是動詞do時，后面出現(xiàn)的動詞用不帶to的動詞不定式。
8) 由and, or和than連接的兩個不定式，第二個to 可以省去：
9) 通常在discover, imagine, suppose, think, understand等詞后，可以省去to be：He is supposed (to be) nice. 他應(yīng)該是個好人。舉例：He wants to move to France and marry the girl. He wants to do nothing but go out. 比較：He wants to do nothing but go out. He wants to believe anything but to take the medicine。
