✔ 最佳答案
很有意思的問題,看你舉的例子,相信你有看過Michael Swan所寫的Practical English Usage
所以,我也就著Michael Swan的講法詳細說多次
首先我們可以在這些結構裏,看to infinitive為一個動作,因為是一個動作故有主詞(施予動作者)和受詞(接受動作者),如:
Japanese is diffcult for Europeans to learn. 這句裏的Japanese是to learn的受詞,故我們可以寫成to learn Japanese
同樣His theory is impossible to understand.這句的His theory也是to understand的受詞,因為我們可以寫成to understand his theory
當主句的主詞也是to infinitive的受詞,我們就可將句子以寫為你以上的結構
Are these berries good to eat?
The letters are ready to sign.
The apple were ripe enough to pick.
The box was too heavy to lift.
但當主句的主詞也是to infinitive的主詞時,我們就不可用這種結構
如我們不可說:Iron is easy to rust.(因為Iron是rust的主詞,因為是主詞我們可以看為Iron rusts),在這種時候,我們大可以用另外的結構說,如:It is easy for Iron to rust.
又如我們不可說:This material is impossible to catch fire(因為This material也是to catch fire的主詞),我們大可說成:It is impossible for this material to catch fire.
簡言之,我們大可將所有句子寫成It is ... to...
若句子可以寫成,如:It is difficult for Europeans to learn Japanese. Japanese因直接在to infinitive 之後時,我們就可以寫成Japanese is difficult for Europeans to learn.
但若句子是:It is easy for Iron to rust,Iron不直接在rust之後,就不可寫成你例子中的結構
希望幫到你!