What exactly is an "adverb"?

From our old grammar textbooks, adverbs are words that describe verbs, adjectives, and other adverbs.

Ex.

He runs fast.

He is incredibly fast.

He did incredibly well.

I didn't quite get what you mean by citing the word "tomorrow." "Tomorrow" has always been a prototypical adverb in that it describes verbs by expressing time. For the same reason, it doesn't use any prepositions before it as well.

---

As for "weekdays," it has often been a noun for me too. But I suppose if "weekends" can be an adverb, why not "weekdays"?

Cool Elf is right: those are all adverbs, because they modify verbs, adjectives, or other adverbs. That is not to say that there aren't different kinds of adverbs; I believe modern Anglo-Saxon syntacticians even use different words for them. But this is the meaning of the word "adverb" as it is commonly used.

As to tomorrow, your example is a just a regular semantic syllepsis, which proves nothing:

I hit the ball and my head.

This doesn't mean that ball and head aren't both nouns. It just sounds odd because the verb hit is used in two slightly different ways.

An adverbial phrase (AdvP) is a linguistic term for a group of two or more words operating adverbially, when viewed in terms of their syntactic function.

Compare the following sentences:

• I'll go to bed soon.
• I'll go to bed in an hour.
• I'll go to bed when I've finished my book.

In the first, soon is an adverb (as distinct from a noun or verb), and it is an adverbial (as distinct from a subject or object). Clearly, in the second sentence, in an hour has the same syntactic function, though it does not contain an adverb; therefore, a prepositional phrase consisting of a preposition and a noun (preceded by its article) can function as an adverbial and is called an adverbial phrase. In the third sentence, we see a whole clause functioning as an adverbial; it is termed an adverbial clause.

So, 'weekdays' is an adverb for the same (syntactic) reason 'soon' is an adverb in the above example.

Reference: Wikipedia, Adverbial phrase

Most anything that answers a “when” question can be roped into service as an adverb, even if it is normally considered a noun or a prepositional phrase.

Q: When are you going?

A: Immediately.
A: Soon.
A: Now.
A: In a while.
A: After I’m done eating.
A: Tomorrow.
A: Next week.
A: Friday.
A: Never.

All these answers are acting like adverbs in this context. But are they really adverbs? What about nouns like tomorrow or next Tuesday? Are those adverbs, too?

The simple answer is “Yes.”

A better answer is “Well sure, sorta.”

But the best is answer is “What’s an adverb?”

And thereon hangs a much longer tale.

---

The problem is, asking whether something is an adverb is a devilishly loaded question. It assumes that there is such a thing as a “real adverb”, which as it turns out isn’t a reasonable assumption at all. There are just words, and words do as they please — meaning, they do as their speakers please. _{^{(Yes, Humpty-Dumpty was right after all. :)}}

Sometimes they happen to do jobs we call adverbial, like answering “when” questions. So, for a short-hand, we call them adverbs there, using a classic part of speech tag known to scholars and school children alike.

Understand that part-of-speech (POS) tags are just an invention. They are sometimes a useful invention, true, but there are not really a necessary one. That’s because words in English are free to fall into whatever slot they want to, to do the needed job. That’s why we end up having so many “this as that” type tags when doing good POS assignment in natural language work on a computer.

One thing that occurs to me is that these “noun-adverbs” (meaning nouns doing an adverbial job) do not appear to admit normal adverbial inflections into the comparative and superlative degrees. Go back to the list of A: ... adverbial answers above and try to inflect them by degree. Sure, you can do something sooner, so that one inflects. But some do not. You are free to “do something tomorrow”, but you may not “do it *more tomorrow”.

Perhaps it bothers you that we have words doing one of the (many) duties of adverbs by answering temporal questions, but which refuse to be roped into another customary adverbial duty, inflections according to degree. Is that perhaps the origin of the question? If so, then the problem is really that we need more distinct parts of speech than the traditional ones.

One problem with assigning POS tags to English words is that this is something of an artificial distinction, the product of artifice alone. All that matters is how a word is used, and even then the granularity of your tag-set varies considerably. In short, it just depends how you slice it.

You will find that the POS tag-sets used by various reference works vary a bit, sometimes a good bit. Even the OED changed a little in how it assigns parts of speech to senses between v2 and v3. For example, many words once marked as a prefix or suffix in the OED2 are now held to be combining forms.

This is especially noticeable when doing syntactic analysis for natural language processing. The parser will make POS assignments to each word in the sentence analysed, and you have to know what each POS tag means.

A particularly common set of POS tags is the Penn Treebank tags. Someone who comes from the school that admits only the seven “classic” parts of speech (^{_{NOUN, PRONOUN, VERB, ADJECTIVE, ADVERB, PREPOSITION, CONJUNCTION}}) may find Penn’s 36 POS tags to be elaborate and useful. But I am not especially fond of them, because they conflate many things that are useful to distinguish in a parse. I prefer the NUPOS tagset, which is a much, much richer tag-set.

If you look at the NUPOS tags for adverbs, you will find that they have a category of adverb called a noun-adverb, meaning a noun used in a slot expecting an adverb, analogously to how a noun-adjective is a noun used in a slot expecting an adjective.

This isn’t anything fancy, and is indeed the very phenomenon we’re discussing here. When we say “Go home”, we find that we are using home, a word normally thought of as a noun, as an adverb. That’s because we are indicating where to go, and where is an adverbial application. If you like fancy words, locatives are always adverbs. ^{_{(And home is a very good example of a locative, and a very popular one historically just as it is today. That’s why the noun for home in Latin, domus, preserved a vestigial locative form, domī, but lost almost all the other distinct locative inflexions for the rest of its nouns.)}}

Where some classical grammars use 7 POS tags and Penn uses 36 of them, NUPOS uses 17 major word classes:

But those 17 are further split up into a set of 34, including things like this:

Even there, we can see that here there is such a thing as an adverb/noun, which belongs to the major class of noun. But there’s more than one way to skin a cat, which is why NUPOS goes much farther, dividing up those 34 major classes into 241 different final POS tags. Here for example are the adverbial NUPOS tags, with illustrative examples:

Yup, that’s a lot of POS tags. But it is useful for people doing NLP to have these nuanced distinctions. It may be useful in other work, too.

So which of those are adverbs? Hard question. Facile answer is that those beginning with av* are. Oh and dx. Maybe some others, too.

See the problem? We’re categorizing things according to their job in the actual phrase, and words in English are super-flexible in their job-duties, much more so than a dictionary’s simple-minded part-of-speech listing suggests.

The best way to remember how an adverb works is very simple: it describes how something is done or where or when, the verb is the action, the adverb describes the action.

The sign is a truncated form of a sentence; this is common. The original complete sentence more likely would have been:

"We are open on weekdays from 9 am to 6 pm."

"We" is the subject, a pronoun used to replace the owners of the store, who are saying something to us. "Are" is the action, the verb. "Open" describes "We", and thus is an adjective. The rest of the sentence is a complex prepositional phrase, at least if you are still learning. It describes how or when something is being done, so it works as an adverb.

The other thing to remember is that much less commonly an adverb describes and adjective (and consequently it NEVER describes a noun, such as the subject or object of a sentence.) It modifies the adjective. It tells us to what degree, how much, what kind of adjective.

The night was surreally beautiful and still.

I am very sick today and cannot come to work.

Remember these rules and you shall never fail to understand. Other than that, a note: Speakers of American English use something called the adverbial genitive more often than the rest of the English speaking world; American English received a large amount of its vocabulary and structure from British English as it was spoken from 1620-1730. The adverbial genitive was more common in England at the time but rarely is used anymore by comparison. So, if an American says "I don't schedule meetings Fridays" he is correct, but using an older form that might not work in a British dialect.

As two professional linguists David Ward and John Lawler, said last week and the week before that, respectively, "Part of speech (POS) is not important: function is important". A nominal adjective is always a noun, even when it functions as an adjective. Tomorrow functions very well and most often as a noun and an adverb of time. I don't know which POS of speech it is. The dictionary calls it both a noun and an adverb based on its function, not its POS.

He's a real nowhere man,
Sitting in his Nowhere Land,
Making all his nowhere plans
for nobody.

Nowhere is an adjective in the first and third lines in these Beatles lyrics, a noun (function: nominal adjective) in the second line, and a locative adverb in the sentence "I have nowhere to go". POS doesn't matter: function matters.

"I'll see you Monday" is merely an elided sentence because grammatically, according to Chomsky, the underlying structure is "I'll see you on Monday", so the preposition in front of the proper noun Monday is missing. However, because there's no preposition and it has the same structure as "I'll see you soon", it seems to function as an adverb of time. Does the underlying structure really exist? Does it really matter? It's both a terminological question and a theoretical question. What linguistic theory or paradigm are we using to parse the sentence: structural, functional, generative, cognitive, or some other? I don't know how many there are, but whatever one's answer, one has put oneself into a box that restricts and, therefore, biases one's analysis.

If we don't all agree to the same definitions and values for the terms we use when describing language, then how can we meaningfully discuss it?

At English Forums: Adverbial Objectives?, there is an article listing quite a few adverbial objectives (nouns used as adverbs, or nouns used as if they were adverbs) by 'paco'. While I'd query the classification of some of his examples as adverbs / adverbial objectives, and the jury is out on whether the nouns have fully converted to adverbs, it's useful reading.