49.不定式作主语
1.不定式(短语)常可用作主语，如:
To see is to believe．
眼见为实。
To act like that is foolish．
这样做是愚蠢的。
2.但在很多情况下，我们都把不定式往后移，前面用先行词it作形式上的主语，这样句子结构会显得更平稳。这时谓语可以是：
1）be+形容词：
It isn't right to talk like that．
这样说是不对的。
It's difficult to master English．
精通英语是不容易的。
2）be+形容词+for引起的短语：
It's hard for me to answer．
我很难回答。
It isn't right for us to stop half way．
我们半途而废是不对的。
3）be+形容词+of引起的短语：
It's kind of you to think so much of us．
难得你为我们这样着想。(=you are kind to...)
It's good of you to come to meet us．
非常感谢你们来接我们。
4)be+名词：
It's our duty to do that．
这样做是我们的职责。
It's a mistake not to help them．
不帮助他们是错误的。
5)其他形式的谓语：
How long does it take to get there?
到那里需要多少时间?
How much did it cost you to send the package?
寄这包裹花了多少钱?