最简单的区别就是：固定搭配不能拆开，动词短语可以拆开，灵活运用。
look for 不能拆开。ask for 就能拆开
比如：look after为及物的短语动词，意为“照顾、照料”，而不是看后面的意思。
look at则不是短语动词，因为look为不及物动词，at为介词，后接名词构成介词短语作目标状语，表示往某个地方看，其意义就是look的意义和at的意义的简单相加。
我们可以说He looked carefully at the map.
但不可以说He looked carefully after the baby. 只能说He looked after the baby carefully.
当我们写定语从句时，关系代词为宾语时look after不可拆分，但关系代词作at的宾语时，look at可以拆分。例如：
可以说：The map at which he is looking carefully is mine.
也可以说：The map which/that he is looking at carefully is mine.
但我们只能说，The baby that/who/whom he is looking after carefully is his sister.
不能说：The baby after whom he is looking carefully is his sister
