（1）句子主语和不定式动词可构成逻辑上的动宾关系。
我们先分析下面这个例句：
She is interesting to listen to.
请暂停，先思考一下这句话的含义。我们给你两个选项：
A. 她对听别人讲话很感兴趣。
B. 听她讲话很有趣。
其实，这句话相当于说“It is interesting to listen to her.”。意思是“听她讲话很有趣”。因此，原句的主语she在逻辑上是作不定式to listen to的宾语。
更多例句：
a.Relativity theory isn't easy to understand. = It isn't easy to understand relativity theory.　相对论不好理解。
b.She is very nice to talk to. = It is very nice to talk to her.　和她聊天很开心。
c.Mary is easy to get on with. = It is easy to get on with Mary.　玛丽很容易相处。
d.English is difficult to speak.　说好英文不容易。
e.Football is very interesting to watch.　看足球很有趣。
句子主语与不定式动词存在这种动宾关系的情况下，我们要注意以下三点：
1）不定式动词一般不用被动式。 （最容易出错）
a.English is difficult to be spoken.*
b.Football is interesting to be watched. *
2）不定式后不能再加宾语。
a.Football is very interesting to watch it. *
b.She is nice to talk to her.*
3）不定式动词所带的介词不能省略。
a.She is interesting to listen.*
b.She is easy to get on.*
（2）句子主语和不定式动词可构成逻辑上的主谓关系。
这一句型中的形容词通常表示人的性格特征或行为表现。如：
brave　careful　careless　clever　considerate　cruel
foolish　generous　kind　modest　nice　polite
rude　selfish　silly　stupid　thoughtful
a.He was kind to help us.
b.The boy was careless to break the window.
这时句子可用以下两种方式来改写：

 
但注意，并不是所有的形容词都能这样改写，这取决于特定形容词的用法。