To hear from you is nice.
To be a good teacher is not easy.
不定式作主语时，为了保持句子的平衡，往往以it作形式主语，而不定式置于谓语动词后。如：It's nice to hear from you.
It's not easy to be a good teacher.
通常用于want, hope, wish, like, need, hate, begin, start, remember, agree, learn, pretend, refuse, manage, help等词后。如：I forgot to lock the door.
Please remember to write to me.
My job is to pick up letters.
He seemed to have heard nothing.
I have two letters to write.
I have a lot of work to do.
通常用于want, wish, ask, order, tell, know, help, advise, allow, cause, force等词后。如： He ordered her to leave at once.
He was forced to obey his order.
He got up early to catch the first bus.
He worked hard to catch up with the other students.
To tell you the truth, I told a lie.
（８） "疑问词＋不定式"结构。 如：
I don't know how to choose them.
I cannot decide where to go.
I decided not to go.
He seemed to have cleaned the room before I came in.
The boy is said to have been sent to the hospital last week.
He was too excited to go to sleep.
He was only too glad to go. (他太高兴了，乐意去)
The book is easy to read.
I have a book to read.
例１ I haven't got a chair ____.
A to sit B for to sit on C to sit on D for sitting
解析：该题选C。不定式to sit on在句中作定语，修饰名词chair. 因为不定式和它所修饰的名词间是"动宾关系"，所以不定式必须是及物动词，故此处on不能省略。
例2 He was made ____. A go B gone C going D to go
解析：该题选D。make sb. do sth. 如果是被动形式，不定式do前的to不能省略。
例3 A new factory is ____ very soon.
A to be built B built
C to build D to building
解析：该题选A。is to be built意为"将要被建"。