free 不能做名词， 所以严格上说 for free是语法错误的，虽然这种用法十分普遍 ( 仅限于informal，非正式场合使用）。
下面是一段国外语论坛对该词组的注释：
Strict grammarians will tell you that for free is grammatically incorrect because free is not a noun, and this means it cannot be preceded by for (a preposition).
In their view, something is sold for nothing or is sold free.
However, through common usage, for free has become acceptable.
所以， 如果要正式书面表达"免费"， 可以直接用free（本身有形容词或副词词性）， 或用for nothing代替。
