一、作主语,可以用it代替,it叫形式主语,动词不定式放在后面叫真正主语1.To learn a foreign language is not easy.It is not easy to learn a foreign language.2.It’s dangerous to drive very fast.二、作表语My idea is to ring him up at once.我的想法是马上给他打电话。如果主语部分含实义动词do,且作表语的动词又是“do”的内容,这时表语不定式的“to”可以省略。如:All I did was wait here.我能做的就是在这里等。 First, the subject, you can use it instead, it is called the subject of form, the verb infinitive on the back called the real subject 1.To learn a foreign language is not easy.It is not easy to learn a foreign language.2.It’s dangerous to My idea is to ring him up at once. My idea is to call him right away. If the subject part contains the semantic verb do, and the verb that is the idiom is the content of the “do”, the infinitive “to ” can be omitted. All I did was wait here. All I can do is wait here.