Ⅰ. 不定式句法功能
1. 作主语：The cat said, “To take roller coaster” is terrible.
不定式短语作主语时，可以直接放在句首，但在很多情况下，尤其是在疑问句和感叹句中，往往放在谓语之后，而用先行代词it作形式主语。
→ The cat said, “It’s terrible to take roller coaster.”
How long did it take you to take roller coaster?
How terrible it is to take roller coaster?
不定式作主语常见句型：
a) It is adj. (easy, important, difficult…) 不定式
b) It is n. (a pity, a pleasure, one’s duty, a shame) 不定式
eg. It’s my duty to teach you how to be a student of No.3 Middle School.
c) It takes/needs/requires some time (hours, months, days, patience…) 不定式
eg. It requires patience to be a good teacher.
2. 作表语：当句子的主语是aim, idea, policy, question, suggestion, wish, task, duty, job, purpose等或者主语是what引导的名词性从句时，后面可以用不定式做表语，用以说明主语所包含内容。
eg. Our most important task now is to make a plan.
注：作表语的不定式都带to，但当主语部分有实义动词do时，to可以省略。
eg. The only thing we can do now is wait and see.
3.作宾语
The cat said “Remember not to take it next time!”.
a) 可以直接用不定式作宾语的动词很多，常见的有：agree, afford, tend, ask, decide, determine, expect, fail, hope, learn, intend, manage, offer, plan, promise, refuse, want, wish等
2005年天津卷12题：I don’t want _____ like I’m speaking ill of anybody, but the manager’s plan is unfair.
A. to sound B. to be sounded C. sounding D. to have sounded
当不定式短语比宾补长时，往往将不定式放到宾补后，而用先行代词it作形式宾语，常用动词有feel, think, find, believe, consider, make等。
The cat felt it terrible to take roller coaster.
b) 不定式一般不作介词的宾语，只有少数介词如but, except等后面可以跟不定式作宾语。一般情况下作介词宾语的不定式都带to，如果but或except所在句子里的谓语动词都是实义动词do, does, did时，通常省略to。
 

本文来自：逍遥右脑记忆 https://www.jiyifa.com/waiyu/yingyuxuexi/18254.html

相关阅读：英语听力训练要讲究策略
句子比单词重要
“谈情说爱”英语篇
实用英语写作技巧6 如何写好段落（Ⅰ）
高中英语不定式和分词表原因的区别