不定式作定语修饰名词或代词时，不定式和它所修饰的名词/代词之间有逻辑上的主谓关系、动状关系、同位关系或动宾关系。不定式通常要放在这些被修饰的名词/代词之后。
1.名词或者代词前有序数词、形容词最高级修饰时，或者被修饰的中心词是序数词、形容词最高级时，常用不定式作后置定语。
It was the first play to win all three of these major awards.
它是赢得所有这三项大奖的第一部戏
He was the last guest to arrive.(主谓关系)他是最后一个来的客人。
2.被修饰词前有 the only,the very,the next等词后用不定式作定语。
The next thing to do is to relax yourself completely
(动宾关系)下一步要做的事情是完全放松你自己。
不定式作定语通常要放在它所修饰的名词之后，表示一个将来的动作，即不定式的动作发生在谓语之后。
【1】修饰主语不定式
A:表示一种将要发生动作且是被动(to be done结构)
The car to be bought is for his sister.
The car which will be bought is for his sister.
要买的那辆车是给他妹妹的。
B: 表示一种将要发生动作且是主动(to be do结构)
The person to meet you at the railway station is called Jack.
=The person who will meet you at the railway station is called Jack.
C:表示一种正在发生动作且是主动(to be doing结构)(不定式动作发生讲话人之前，且一直都在进行之中)
The boy to be sleeping in the room is my younger brother.
=The boy who is sleeping in the room is my younger brother.
D:表示已经事先已经完成了动作(to have been done结构)
The building to have been built is our teaching building.
=The build which has been built is our teaching building.
E:表示从过去到现在一直都进行的动作(to have been doing结构)
The person to have been watching TV in the room is my father.
=The person who has been watching TV in the room is my father.
【2】修饰表语的不定式
A:(表示一种习惯上的泛指或经常反复发生动作)
She is always the first person to come and the last person to leave.
B:表示一种已经完成动作
He was the first person to arrive .
=He was the first person who arrived.
【3】修饰宾语的不定式
A:(表示已将发生的动作且是主动)
I have three letters to post this afternoon .(表示自己邮寄)
今天下午我有三封信要寄
B:(表示已将发生的动作且是被动)
I have three letters to be posted this afteroon.(表示别人邮寄)
C:He need something to eat .(表示自己吃)