在《新编英语语法教程》学生用书第6版（最新版，上海外语教育出版社）223-224页：

2)主句主语是不定式结构的逻辑宾语

分2种情况：一种是能以不定式结构作主语并能转换为先行it-结构。

例如：That question is difficult to answer = to answer that question is difficult = It is difficult to answer that question.

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。例如：

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.

说这些句子都不能以不定式结构作主语，例如不可以说To taste the coffee is bitter, 不可以说To look at them is very attractive等。

我很不明白，为什么不可以说To taste the coffee is bitter, 不可以说To look at them is very attractive等呢？ 麻烦老师详细解释一下。

谢谢！