"I wish for a rest now": what does "now" modify?

The phrase "I wish for a rest now" could be interpreted to mean that at the present time (ie, "now") you are wishing for a rest (presumably beginning immediately, if not sooner), or it could be interpreted to mean that you have a wish that at the present time ("now") you were resting.

The difference in the two meanings is certainly subtle, and, some would argue, inconsequential, since the effects (such as they are) of the two wishes are identical.

The argument that "a rest" is not a verb and therefore does not couple with an adverb is a technicality, of course. If the statement were reworded as "I wish to rest now" then "rest" becomes a verb and the technicality evaporates.

Rewritten this way you could further rewrite as either

I wish to now rest

or

I now wish to rest

allowing you to be more explicit as to the meaning. But neither rewrite is as satisfying (and restful) as the original, and thus such a rewrite would only be appropriate if there was some need to distinguish between the two (nearly identical) meanings.

I agree that "now" modifies "a rest". To make grammatical sense of it, we can assume that "a rest" is the remnant of an understood verb phrase "(to take) a rest". (I don't know where the "for" comes from.)

I am led to this analysis by considering a similar construction, "I wish for an immediate rest." Evidently, "immediate" could not go with "wish", because it is not the wish that is immediate. I think it is also possible to have an adverb form of this: "I wish for a rest immediately."

You could also get "I wish for a rest soon," where "soon" clearly concerns when the rest comes, not when I do my wishing.

I would say that in terms of meaning we tend to associate 'now' with 'rest' in that sentence.

In terms of grammar we are constrained to making the adverb 'now' refer to 'wish'.

The reason is that you cannot use 'now' as an adjective. You cannot say "I want a now rest." However you can say "I want a rest now." which in terms of grammar is equivalent to "I now want a rest."

I sympathise with your point of view.

"... I think that the adverb "now" modifies "rest"." It doesn't.

Here's why:

1. What you're doing now is wishing, which is the verb.
2. Although "rest" can function as a verb, it's a noun here, as it is "a rest".
3. Once you realize the above, you will be able to see why "now" doesn't modify it. But if you believe an adverb (such as "now") can modify a noun, then there's a problem.

What you're actually doing now is wishing, and that's why "now" modifies the verb, "wish".