不定式使用的具体情况：
1、作定语。...的...
the ability to write; the chance to go abroad; the atmosphere to practise our oral English; the potential to treat diseases.
不定式自带逻辑注意（逻辑主语隐去了，可以扩展为不定式的复合结构，即for somebody to do something）。现在分词作定语逻辑主语是被修饰的名词。
比较：the boy sitting under the tree
2、主系表后的状语，修饰、补充表语。表方面，原因。
The computer is easy to operate.
I’m glad that you can come.
I’m happy to meet you here.
3、作主语、宾语、表语、主语补语。
To see is to believe.
They offer to help her out.
This allowed him to have a good sleep.
使让动词后的宾语补语省去不定式符号to。
4、目的状语。一个动作，带有一个目的。
He works very hard to enter a good university.
To enter a good university, he works very hard.