my wife must see it first .my wife must first see it .这样子可以吗?为什么first放在后面.

状语,一般后置.
如：
I get up early.我很早起床.
Please call me as soon as possible.请尽快给我电话.
My wife must first see it.不能说错.但这个状语换了位置,比普通意思要多点儿话外音,强调first.送上这句总结：：：：：副词修饰形容词,副词时,副词在前面, 而被修饰的词在后面。