一、动词不定式作主语

1. It is important to protect the environment. （保护环境很重要）

2. The problem is to find a solution quickly. （问题是如何快速找到解决方案）

二、动词不定式作宾语

1. I want to learn English well. （我想学好英语）

2. She decided to quit her job. （她决定辞职）

三、动词不定式作宾语补足语

1. The teacher asked him to finish his homework on time. （老师要求他按时完成作业）

2. The company hired him to design the new product. （公司聘请他设计新产品）

四、固定句型结构

1. It's one's turn to do sth. （轮到某人做某事）

2. It takes sb. some time to do sth. （做某事需要某人一些时间）

3. It's time to do sth. （该做某事了）

五、其他用法

1. 动词不定式作目的状语：

I want to learn English well so that I can travel around the world. （我想学好英语，以便环游世界）

2. 动词不定式作条件状语：

I will call you when I arrive. （我到达时会给你打电话）

注：部分例句中，it作形式主语，真正主语为动词不定式（如：It's important to protect the environment. 中真正主语为to protect the environment）。