(3)由⑧⑨句可知，不定式在介词but后面时，如果but之前有行为动词do的各种形式，则介词后的不定式不带to，否则带to。

　　一、动词不定式

　　(一)动词不定式的时态和语态

语态

时态　　 主动 被动 一般式 to do to be done 进行式 to be doing / 完成式 to have done to have been done 　　1．不定式的一般式：表示不定式的动作与谓语动词同时(几乎同时)发生或在谓语动词之后发生。

　　I hope to see you next week.

　　我希望下周能见到你。

　　I'd like to be told what's going on.

　　我希望被告知正在发生什么。

　　2．不定式的进行式：表示不定式的动作正在进行且与谓语动词同时发生。

　　I happened to be watching TV when she called.

　　她打电话来时，我恰巧正在看电视。

　　3．不定式的完成式：表示不定式的动作发生在谓语动词之前。

　　They seem to have cleaned the house.

　　他们似乎已经打扫过这个房子了。

　　The factory is reported to have been burnt down.

　　据报道这家工厂已经被烧毁了。

　　[即时演练1]　用所给词的适当形式填空

　　①He pretended to_have_fallen (fall) asleep when little Tom came to him.

　　②The student seems to_have_caught (catch) a cold because of the bad weather.

　　③He seems to_be_thinking (think) about the question right now.

　　④The question to_be_discussed (discuss) tomorrow is about the pollution.

　　(二)动词不定式的语法功能

　　1．作主语

　　To master a foreign language is necessary.

＝ It is necessary to master a foreign language.