一、当to do不定式作主语
1、一般用it作形式主语，不定式后置，具体的.句型为It's adj/noun (for sb) to do。
例如：It's a pity for you to drop out of school.
真遗憾，你竟然辍学了。
It's a good idea for you to eat less fried food
你少吃点油炸食品是好想法。
二、当to do不定式作宾语
1、谓语动词＋动词不定式（多数动词属于此类动词）
例如：Some students hate to study English.
有些学生不喜欢研读英语。
He pretended to be a good man.
他假装是好人。
2、系表结构+动词不定式
例如：I am glad to see him.
我很高兴看到他。
His health is likely to get worse.
他的健康可能会恶化。
三、当to do不定式作宾语补足语
1、具体结构为：主语＋谓语＋宾语＋动词不定式
例如：The teacher urged his students to prepare for the exam.
老师劝他的学生准备考试。
Their help enabled me to finish the work with ease.
他们帮助我，使我能够顺利地完成工作。
2、当动词不定式做make, have, let, hear, see, feel等动词宾语的补足语，要省略“to”
例如：My father will probably have Tom paint the house green.
我爸爸很可能叫汤姆把房子漆成绿色。
I felt my heart beat violently.
我感觉到自己的心跳得很厉害。
四、当to do不定式作目的状语
1、具体结构为：in order/so as + 动词不定式
例如：He must work very hard in order to support his large family.
他必须拼命工作以养活他的大家庭。
Listen carefully so as to follow the teacher.
要仔细听才能听懂老师讲的课。
2、具体结构为：be used + 动词不定式
例如：The book is used as a reference book.
这本书被用来做参考书。