adverb clauses

I have a hard time deciding which word an adverb clause modifies in a sentence, examples:

1. Buy that coat now because it might be sold tomorrow.

(A: because it might be sold tomorrow modifies the verb buy)

I chos the adverb "now" because I thought the main emphasis/reason of the surbordinate clause laid on "tomorrow". If the adverb clause said: "because you need one", I would go with "buy". Compare:

Since I can spare only a few minutes, please be brief with your presentation.

(A: Since I can spare only a few minutes modifies the predicate adjective brief) The word modified is not the verb "be" rather it's the pred. adjective "brief", I'm assuming that's because its indication of time scrunch.

So my impressioin was knowing which word, along its part of speech, is modify has a lot to do with what the adverb clause main focuse is, but I must have been mistaken. I'm not doing well in the practices. Could someone be kind enough to give me some guidance in deciding what to pick, is there a rule of thumb? Thanks a lot.

Note: the grammar rules say: An adverb clause is a dependent clause that modifies a verb, adjective, or another adverb. It usually modifies the verb.

Regards

Raen