look for his pen 为什么不能说look his pen for

look for his pen 为什么不能说look his pen for

因为look for 是动词与介词组成的短语，不能分开。
所以只能说 look for his pen ,不能说look his pen for。
类似的有look after , take care of 等。
拓展：
动词与副词组成的词组，代词作宾语时要用宾格放词组中间，名词作宾语时可放中间也可放在后面。
如：

The radio is too loud , please turn it down . ( 不能说 turn down it )
Please turn down the radio.
 =Please turn the radion down .

类似的常见的用法还有：
turn up , turn on , turn off , give up , take off , put on ,write down ，take away 等

look for是一个短语，表示“寻找”

look for为动介词组，不宜分离。
look for 为固定搭配