I was watching a movie last night and a character, when asked a question, replies, "God only knows". Is that the correct usage? It sounds to me as if God just knows stuff and can't do anything about it, if you know what I mean. Isn't "Only God knows." more appropriate?
It would be, if English had a more rigid word order. Or in written English, where the stresses and intonation of the spoken language do not always translate well, even when typographical cues like italics are employed.
In the case where one is stating that God merely knows, there would be a subtle stress and (usually) a rise in intonation over the word only (which would be more pronounced if one were being explicit, as when countering a suggestion that God should have been able to do something about the situation).
Without the stress (or with the stress on God instead), the two formulations are equivalent in meaning. Well, sort of equivalent. God only knows is an idiomatic phrase meaning more-or-less I don't know, and I doubt that anyone else does either. The alternative, Only God knows, is a little more philosophical, theological, or at least religious in tone, and means something more along the lines of That's beyond the ken of mere mortals. The strict literal meanings of both formulations are the same, but the implications are different because the more archaic inverted version has become idiomatic and lost its theological implication. An Anglophone atheist would have no more trouble saying God only knows in an ordinary conversation than, say, God damn it, but would only use Only God knows ironically.