the policeman has caught the murderer alive.为什么用alive

alive 关于用作定语
通常不放在名词前作定语,但可作作表语或后置定语.如：
He must be still alive.她一定还活着.
He’s the happiest man alive.他在世界上最幸福的人.
Although he is old,he is very much alive.虽然他很老了,但还很有生气.
注意：
但是若本身带有修饰语,则可以用作前置定语：
a really alive student 一个十分活跃的学生
a really alive town 一个十分热闹市镇
除用作表语和有时用作定语外,alive 有时还用作状语或补语.如：
He was buried alive.他被活埋了.
The spy was caught alive.特务被活捉了.
I’m afraid he can’t come back alive.恐怕他不能活着回来.