>It is perhaps not the best of ideas to illustrate a claim regarding
>word order with a set phrase like that.
That's an excellent example of where "only" belongs in this UNAMBIGUOUS s-e-n-t-e-n-c-i-a: right before the verb. By the way, set phrases explain a lot.
it is unambiguous only because it is always remembered and reproduced as a whole. One thing you don't seem to notice, though, is that it is not always a sentence - it can be followed by a wh-clause (as in the song by the Beach Boys - and Judy Tzuke). The reason why "God only knows how I feel about you" does not look ambiguous is just that 'God only knows' has a set meaning assigned to it. However, "Tom only knows so much", does not mean 'all others know more', but 'Tom knows little'.
You'd better have a darn good reason to move "only" from before the verb; you could do it either to avoid ambiguity or to emphasize something that otherwise poor wording won't do for you.
>Would you say "Tom only knows (that)' or 'Only Tom knows (that)'?
You can bet your liver that "Tom only knows that (period)" sounds better than "Only Tom knows that (period)." That's how it wants to sound.
You could come up with a better example to gloss over your ideas next time.
Cutting it short was too easy.