Not a very good idea to use dictionaries for parts of speech. They're good for lexicography, not for grammar. Here's why. You'll get different results if you look in a modern published grammar.

See also this at ELL where 'where' is discussed as a preposition

Do you have any evidence for your claims here? Are there any adverby properties of now that you can show us when you say now is an adverb? Are there any conjunctiony properties that you can show us when now is a conjunction? Are there any nouny properties that you can show us when now is a noun?

@Araucaria Evidence that "now" is an adverb? (a) Every dictionary I checked says its an adverb. No dictionary I checked calls it a preposition. You say you reject dictionaries as a source for parts of speech. Well, hmm. I'd certainly agree that a simple classification like you'll find in a dictionary might be inadequate for tricky or specialized cases. But most people accept them for routine cases, which is what we're talking about here. ...

... If your position is, "I believe X is true. Any source that says X isn't true I refuse to acknowledge as authoritative", then of course it's impossible to prove you wrong.

(b) "Now" meets the most elementary definition of an adverb: It modifies a verb. If you want to get into some complex linguistic debate, I just don't see how that would help English Language Learners. We're not talking about sophisticated, complex cases here. We're talking about the elementary rules for people trying to learn the language.

@Jay No, my position is that no argument for now being an adverb has been cited. The sources that I cite put forwards examples from the grammar that support their analyses. I don't think that you'll find that in a dictionary. Secondly, some famous dictionaries haven't updated some of their entries for a hundred and twenty-seven years. Not likely to be up with the new discoveries.

@Jay Now does not modify verbs. It can modify verb phrases, but that's an entirely different kettle of fish. That schoolboy definition of an adverb has no place on a website for linguists and serious enthusiasts of language. Adverbs can modify a huge number of different types of phrase. Modifying a verb does not make something an adverb. Neither does not modifying a verb either. Note that adverbs like very do not freely modify verbs. Also note that now can post-modify nouns.

@Jay No, that would be our sister site ELL. But in any case giving learners daft rules that mislead them will only help to ensure that they make mistakes. For example the specialised adverb right doesn't modify adverbs, but does modify prepositions. So if you tell a learner an 'elementary rule' like 'now is an adverb' they will naturally assume that you cannot say right now - which is of course rubbish.

@Araucaria Oops, I was thinking we were on ELL -- got here from a link on ELL. I concede that point.

@Araucaria Yes, adverbs can modify things other than verbs, but that's not the point here. I don't see how you say that "now" can't modify a verb. "Sit down now." "I will try now." Etc. How would "now is an adverb" rule out "right now"? "I will try right now." By the elementary rules, "try" is a verb, "now" is an adverb modifying "try", and "right" is an adverb modifying "now". Clearly "right" is modifying "now": Not just a general "now", but "right now".

@Jay Exactly, sure and fast evidence that now is not an adverb :)[Compare right immediately or right currently or right soon - right doesn't modify adverbs]. If you use a verb without a complement you won't be able to distinguish between the verb and the verb phrase. Try "put your plate in the dishwasher now" <--- is now modifying put or put your plate in the dishwasher?

@araucaria Hmm, by that reasoning, "tall" is not an adjective, because phrases like "tall sleep" and "tall electricity" don't make sense, so therefore "tall" does not modify nouns. I don't get your point on the dishwasher example. Consider "Put your plate in the dishwasher carefully." The exact same issue. Is "carefully" not an adverb?

@Jay No, because "tall" is not just an intensifying word it has a well-defined lexical meaning. And if we say "tall electricity" it may not make sense but it doesn't grate on our grammar ear. In contrast we can understand "right soon", but it isn't grammatical. My point about now is that it can be an adjunct in VP and clause structure, just like other preposition phrases and adverb phrases, but that the schoolkid adage that adverbs are words that modify verbs is silly. It shouldn't be advertised on this site.

@Araucaria The difference between right now and *right immediately is interesting. It would be decent evidence as part of a larger argument that now is not an adverb (or at least not the same subclass of adverb as immediately). But it's no evidence that now is a preposition. It's the same class as then and here I think, which are also often called adverbs...

@curiousdannii Ah, but those are all intransitive prepositions too! You might find some of these three posts interesting back, there, out there.

@Araucaria Intransitive prepositions? Forgive me but I have to say that sounds like nonsense to me!

@curiousdannii It's as old as the hills - well a hundred odd years old. Been accepted by many linguists since the seventies. Just because some idiots decided that this word family often came before nouns doesn't mean that the whole word class does. Imagine what would happen if you tried to divide up verbs according to what Complements they take. It's a dumb, dumb stupid, idiotic nonsensical, wrong-headed and childish way to approach a word class, to try and categorise them by what words come after them!!

@curiousdannii It's just very difficult to come to terms with after being taught that for so long.

@Araucaria You're completely wrong about my approach to word classes. I define adpositions, as I assume most linguists do, as a class of words which mark the semantic roles of constituents, functioning just like case affixes do in other languages. An "intransitive" adposition would make just as much sense as a case affix without a noun! (NB I would not call a preposition with an elided complement intransitive.) Now Alan Munn below says that now is a pro-preposition, which makes a great deal of sense to me, but that is a completely different thing from an intransitive preposition.

@curiousdannii Sorry if I gave the impression that I was talking about you personally. I wasn't. However, what kind of case analogy is being ascribed in until recently? How about in In depends on who's coming? How come traditional grammar still recognises those as prepositions when there's no noun there?

Let us continue this discussion in chat.

@curiousdannii I'm free in about 45 mins. Got to go back to class right now. But will do.

@Araucaria , you're problem with right now is that you're expecting right to behave with every adverb as it does with now. It doesn't. It collocates only with certain adverbs, one of which is temporal (another is there). Second, your example of put the dish in dishwater now is misleading because verbs of "conveyance" require an object & a locative element; because of this, it is impossible to remove "the dish in the dishwater" and analyze the verb & now, &, frankly, unillustrative of anything. Third, you can replace now with other adverb(ial)s (at noon, today/tonight, later,...

[cont.] when you're finished with your meal). Fourth, now can function (temporally) only in terminal and introductory position but not always in medial position put [*now] the dish [*now] in dishwater. *Now functions introductorily as a nontemporal disjunct as well as a regular adverb. Fifth, prepositions as a class require certain properties, as all others do, so their restriction to preposition+noun (or noun phrase) creates a well-defined boundary. Now, if I had to analyze until recently, I'd say that it might be an elliptical adverbial clause, but that seems somewhat dissatisfactory…

[cont.] Sixth, verbs have been categorized by their complementation, & I certainly don't think it's stupid. Seventh, the difference between after & on is that one is a nominal clause which acts as a noun whilst I can only call the other an "independent clause".

@JasperLocke I don't have a problem with now ! I merely concede that it's a preposition in deference to two of the three greatest grammars of English ever written. Are you trying to say that adverbials are adverbs, i.e. that nouns and preposition phrases are adverbs? I don't get the second half of your first comment. It's a bit difficult to proceed without clearing that up.

@Araucaria 1) I said right now not now. 2) Now is not a preposition; it's an adverb that answers the question when. The distinction between preposition and adverb disappears when prepositional phrases are reduced to prepositions. 3) The distinction between adverb and adverbial is unnecessary. 4) The verb put, which requires an object (the dish) and locative element (in the dishwater), cannot be analyzed alone with now as you could with Run the dog to the store now -> run now (verb+now).

@JasperLocke with the greatest respect, you don't seem to fully get the difference between grammatical relations and parts of speech.

@Araucaria with the greatest respect, you're a poltroon. I understand perfectly well that linguists think of adverbial as a functional category and that adverb as a formal one. I just reject it.

@JasperLocke Thank you very much! OK, do you reject the difference between being wooden and being a plate for example? How about the difference between being metal and being a plate?

@Araucaria Asking for the difference of being wooden/metal & being a plate is nonsensical because one's composition, the other shape (more or less). A better analogy would have been metal vs. wooden plate. I'll tell you why the term adverbial is useless: when grammarians set out to define what part of speech something was, they had to think about them functionally as well as formally. Before an adverb can be talked about, one needs to know what an adverb is, what its properties are, & what it does. As such, the distinction is frivolous.

@JasperLocke That's the point though. Deciding that something's wooden or metal is like finding out whether it's a preposition or a noun etc. Knowing whether it's a plate or a stool or a fork or a window pane, is knowing its function. It's knowing how it's being used. Similarly something may have the function of modifying another chunk of words, for example a verb phrase or a clause (in which case we might call it an Adjunct, or an Adverbial), but it might be a noun or adverb or preposition (or a larger phrase with one of those words as a head).

@JasperLocke Knowing the POS tells us what kind of words might be used to modify it, what kinds of complements it might have, whether we can use determiners with it, how it will inflect, what it will inflect for, what other types of function we'd expect to see the word/phrase in, and so on and so forth. But if we don't differentiate between modifying something and being an adverb, or being the complement of a preposition and being a noun then we can't do these things.