一：主句主语是不定式结构的逻辑主语
能用于这种搭配关系的形容词都是一些动态形容词，大体可分为四类：
1：表示喜怒哀乐的形容词+不定式。
eg:
He is glad to help others.
We were sorry not to be there .
She wasn't content to live a quiet life in a small town.
John was very pleased to be given the job.
I was surprised to see how angry he was .
2:表示运气好坏的形容词+不定式。
eg:
He was lucky to find it.
She was unlucky to spoil it.
He was fortunate to escape being injured.
3:表示心理状态、个人态度的形容词+不定式。
eg:
He is anxious to see her .
He is not very keen to think about it .
I'm determined to finish the work tonight .
We are ready to make the attempt.
如果主句主语不是不定式结构的逻辑主语，就得用for将不定式的逻辑主语引出。
eg:
I'm quite willing for your brother to come with me .
I'd be afraid for her to go swimming alone.
4:表示性格特征和行为表现的形容词+不定式
eg:
He is foolish to meet her again.
She was careless to break the cup.
They were thoughtful to come home early.
She was stupid to make such a mistake.
You are kind to say so .
这类句子都可以转换为 it-句型：
It is foolish of him to meet her again.
It was careless of her to break the cup.
It was thoughtful of them to come home early .
It was stupid of her to make such a mistake .
It is kind of you to say so .
二：主句主语是不定式结构的逻辑宾语
有两种情况：一种情况是，能以不定式结构作主语并能转换为先行 it-结构。
eg:
That question is difficult to answer .
=To answer that question is difficult.
=It is difficult to answer that question.
能用于这种结构的形容词通常都是一些静态形容词。
eg:
This car is expensive to overhaul (检修）
Duck's legs are nice to eat .
That book is interesting to read .
Dirty water is nasty to drink .
A tractor is useful to have on a farm .
She is pleasant to talk to .
The river is dangerous to bathe in .
This bed is comfortable to sleep on .
That place is exciting to visit .
另一种情况是，不能以不定式结构作主句主语，因而不能用先行it-句型予以转换。
eg:
The coffee is bitter to taste.
They're both very attractive to look at .
These oranges are very tasty to eat .
The grass is very smooth to walk on .
This map is clear to read .