I recently had a conversation about the Spanish word "ahora", in which my conversant claimed that "ahora" is always an adverb, and never a noun.
This lead me to investigate the part of speech of similar words, both in Spanish and English (my native language). According to dictionary.com, "now" can be a noun, as in:
the present time or moment:
"Up to now no one has volunteered."
What makes "now" a noun in this context? In the Spanish equivalent of the example ("Hasta ahora…"), "ahora" is considered an adverb by the dictionaries I checked with.
The gist of the question: How can I know when now (or any word, for that matter–especially one which is commonly an adverb) is a noun? What test can be applied?
In the example quoted above ("Up to now…") it is not at all obvious to me why 'now' should be a noun while with, say "Until tomorrow", 'tomorrow' is an adverb. They seem like the same grammatical construct to me, so I would (apparently quite naively) expect the same part of speech to follow "Up to" or "Until."