Is "now" a "preposition"?

"Now" is not a preposition. It is most commonly used as an adverb. "We are leaving now." "Now" modifies "leaving". It says when we are leaving. It is a word that modifies a verb, and is therefore an adverb.

It is also used as a noun. "Now is a good time to start". "He lives in the here and now." It refers to a specific time.

It can be used as a conjunction. "Now listen to me." "Now, here's the interesting part."

I just checked several dictionaries and some mention it can also be used as an adjective. "The band has a really now sound." That's very informal though.

None says it can be a preposition. There may be some sentence you can construct where you use "now" as a preposition and it makes sense. I can't think of one off the top of my head.

I see that one of the answers on the question you cite refers to "right now" as a prepositional phrase. I think that's incorrect. I'd call it an adverbial phrase -- precisely because it does not include a preposition. Note that in the comments following, many dispute calling this a prepositional phrase. I haven't checked the references given. If some linguists are redefining "preposition", well, let's just say that that is not what I or my kids were taught in school, nor what any dictionary I've checked says.

now is not a preposition, but it can be analysed as a pro-preposition

To begin, we must define what a preposition is. Evidently there is some debate, but the definition I would use is that it is a word class used to mark the semantic roles of constituents. Prepositions (and postpositions in other languages, together called adpositions) are only one strategy of marking semantic roles. Other languages use case instead. If you're familiar with mostly European languages, then you'll probably only know of languages with a small handful of cases, which aren't enough to mark very many semantic roles; in such languages the cases generally only mark the morphosyntactic alignments (subject, object etc.) But other languages have far more cases, such as Tsez, which Wikipedia says has 64 cases.

The predicate structure of a preposition has one complement, which is most commonly a noun phrase, but can be an adjective phrase, a clause, another preposition phrase, and probably more in rare cases. I consider the predicate structure to remain the same even if the complement is elided, such as in these examples:

Mark has a lot to be happy about
Who were you just talking to?

Those prepositions are elided because English syntax says they should not be repeated, but they still function to mark a semantic role, and listeners understand that the role is being assigned to a constituent which occurs earlier in the sentence.

Now is different. It can never have a complement. It does not mark or assign a semantic role to anything. Some other posters in this question call now an "intransitive preposition", but that's a nonsensical term in my mind. (I would similarly distinguish between true intransitive verbs and transitive verbs with an elided object.)

So what is now? I think it can be analysed as a pro-preposition, an instance of a pro-form. (Thanks to Alan Munn for this insight!) A pro-form is a word which stands in for a phrase. We are all familiar with pronouns, which stand in for noun phrases, but there are arguably pro-forms for all phrase classes. Now, then, and even perhaps words like yesterday stand in for temporal prepositional phrases like after I arrived. But just as pronouns are not nouns, and fail to meet a noun's diagnostic features, like combining with adjectives or determiners, a pro-preposition is not a preposition. Nouns and pronouns are grouped together as part of a greater word class, the nominals. In some languages this includes adjectives and agreement markers. I'm not aware of a corresponding term for prepositions, so I would like to suggest that we call the superclass of prepositions and pro-prepositions the prepositional word class.

Other posters have presented some syntactic evidence to argue that now is not an adverb, chiefly focused on whether it can combine with right or not. That's not a debate I want to enter in to, but I'd like to offer an explanation for why now has traditionally been identified as an adverb. Now and the prepositional phrases it stands for are generally temporal adjuncts, an optional part of clause giving further details. Most adverbs like quickly are used as clause adjuncts, so it's understandable why now would be grouped with them. So while now may not be a real adverb, it definitely has an adverbial function.

Syntactic criteria for prepositions

Although definitions of parts of speech sound nice, they really don't capture the generalizations correctly much of the time. So in order to answer the question of whether now is a preposition, we need to determine some syntactic diagnostics for distinguishing prepositions from what they are most often confused with, adverbs.

So we want the class of prepositions to be those words that behave syntactically like other words in the same class, and differently from words in a different class. Crucially, in the cases of adverbs and prepositions we can't just look at the places where they both can occur, since these positions will never be probative. In particular, since prepositional phrases and adverbs can both modify verb phrases, we can't use "modifies a verb phrase" as a useful definition of an adverb.

The first thing we need to get rid of is the traditional notion that a preposition is something that always takes an object. This is part of the reason why lots of prepositions get classified as adverbs, but it really doesn't make a lot of sense. To see why, consider the following prepositions:

1. John went in the house. / John went in.
2. John ran out of the house. / John ran out.
3. John looked down the street. /John looked down.

In each example the same word, with the same meaning, can appear both with an object and without an object. So defining "Preposition" as something that takes an object (making the other versions adverbs) makes very little sense, and doesn't capture the pattern very well. We don't say that an optionally transitive verb becomes something other than a verb when it is used intransitively, so we shouldn't say that an optionally transitive preposition becomes an adverb when used intransitively either; it's just a preposition without an object.

Now when we look at these prepositions, we find that both the transitive and the intransitive ones can be modified by right:

4. John went right in the house. / John went right in.
5. John ran right out of the house. / John ran right out.
6. John looked right down the street. /John looked right down.

Adverbs behave differently from prepositions

This property of being modifiable by right is a property of prepositions in English, and fails when applied to things that are uncontroversially adverbs: (I'm excluding the dialects in which right can modify adjectives and adverbs with a meaning of "very"; in those dialects this test is weaker, obviously.)

7. John ran quickly.
8. *John ran right quickly.
9. John eats apples frequently/often
10. *John eats apples right frequently/often.

In addition to being not modifiable by right, adverbs are impossible as the complement of the copula verb be. This distinguishes them from almost all other categories in English. But all of the intransitive prepositions are perfectly fine after the copula.

11. *John is frequently/quickly/often.
12. John is out/in/outside/inside.

"Now" is a preposition

We now have some tests that we can use to determine whether now behaves more like an intransitive preposition or like an adverb. With both of these tests, now behaves like a preposition and not like an adverb.

13. John is leaving right now.
14. *John is leaving right immediately.
15. The meeting is now.
16. *The meeting is immediately.

We can also show that immediately can modify temporal prepositions, while now cannot:

17. The meeting is immediately after the game.
18. *The meeting is now after the game. (≠ "immediately after the game")

So we have little evidence that now is an adverb. The fact that dictionaries call it an adverb is simply because they call all intransitive prepositions adverbs, but this is not supported linguistically, which is why (almost) all linguists (at least ones who study syntax) will tell you that now is a preposition.

A more nuanced version

Strictly speaking, what we have shown in the discussion above is that now has the same distributional properties as a prepositional phrase, and in this respect is distinct from an adverb phrase. What it doesn't show exactly is that now is a preposition as a word.

Pronouns and nouns

To make sense of this difference, we can consider the difference between pronouns and nouns:

19. John met a man/the man/that man.
20. John met Bill
21. John met him.

Traditionally we say that phrases like a man, the man, that man are noun phrases, because they contain a noun and that seems to be the most 'important' element of them. Now we know that pronouns like him substitute for noun phrases, but we typically don't want to analyze pronouns as nouns. Conversely, even though proper nouns like Bill don't have usually have determiners, we don't want to say they are not nouns. The reason for this is that pronouns cannot themselves take determiners, but proper nouns can:

21. *The him/he that is in my stats class is nice.
22. The Bill that is in my stats class is nice.

Some syntacticians (including myself) take this fact (along with some others) to be evidence that what we traditionally call noun phrases are in fact determiner phrases, and pronouns are in fact "intransitive" determiners, while proper nouns are nouns with a null determiner. This means that pronouns, proper nouns, and traditional noun phrases with determiners will all have the same phrasal distribution.

There are a couple of simple argument for the view of pronouns as determiners. First, it allows for a simple characterization of the demonstrative determiners: (this may be an oversimplification, but I will leave that aside.)

23. I liked that book.
24. I liked that.

We can treat that as an optionally "transitive" determiner: in 23 it takes an NP and in 24 it doesn't.

Second, in many languages the definite determiner forms are morphologically identical to the weak pronouns. This is true e.g. in French and Spanish, for example (data from French, but equivalent data can be constructed in Spanish).

25. Je lis le livre/les livres. I read the.MSg book/the.MPl books "I read the book/the books."
26. Je le/les lis. I it/them read "I read it/them."

Additionally, the idea that proper nouns are nouns with null determiners receives more support because in many languages proper nouns show up with a definite article. (e.g., Portuguese, Greek, and also dialects of Spanish and German.)

From this discussion we can conclude that pronouns are not nouns themselves, but are more likely "intransitive" determiners, and this is what allows them to be pro-forms for other Determiner+Noun combinations.

Applying this analysis to Prepositions

Assuming this analysis of noun phrases as DPs is correct, we can apply a similar logic to the analysis of prepositional phrases. We know that there are pro-prepositional phrase elements, like there and then. These elements can also be modified by right (right there, right then) and so pass our test for prepositional phrase.

27. John put the book on the shelf/under the table.
28. John put the book there.
29. John left at noon/on Tuesday.
30. John left then.

It's obvious that now is much more like these elements as well, since it is anaphoric to the speech time, just like then is anaphoric to some non-speech time.

31. I have to leave at 3:00 PM.
32. That's now!

So just like a pronoun substitutes for a Determiner+Noun combination but is not itself a noun, but rather a determiner, we could say that there and then (and therefore now) are pro-prepositions, i.e., an element analogous to the determiner in a determiner phrase. For concreteness, let's call this category R and therefore there and then and now are RPs.

This may well be the correct way to analyze these elements, but it does come at a cost: if we want to say that the pro-forms are not prepositions themselves, but of category R, we are now committed to saying that all the things we thought of as prepositional phrases are RPs with null R elements. Unfortunately we don't seem to find any R-words that combine directly with PPs (i.e., there is no R-element that is analogous to the, for example.

I'm actually quite sympathetic to this kind of analysis on theoretical grounds, but I don't think it gains us very much descriptively, which is why I think that on descriptive grounds, calling there, then and now prepositions captures their syntactic behaviour quite nicely.

'Now' is not a preposition, conjunction, or adverb. It is actually a pronoun whose antecedent is the current time. It only looks like an adverb because times often appear as adverbials, naked of a preposition {we never say 'at now'}.

'Now' can be paraphrased as 'the present time', just as 'today' can be paraphrased as 'the present day'. These words are almost always used adverbially, meaning that a temporal reference is being given for the action. The reference is an elided '[at]', and the referent is the pronoun (or rather the time that corresponds to the pronoun); thus 'now' means '[at] the present time'. Here are some examples to demonstrate that 'at' is not an integral part of the paraphrase, and so 'now' is not a preposition (per Huddleston and Pullum), but it is a pronoun:

Now is a good time to go.

Until now, I was a jogger. {'until the present time'; not 'until at the present time'}

From now on, I will run.