There aren’t a lot of hyphen rules in English, but there are some traps in them for the unwary.

Hyphens do two main jobs. They join together (some) compound nouns and compound adjectives. Let’s discuss those, and also some things that hyphens shouldn’t be used for that seem similar.

Compound nouns

Compound nouns are made up of two or more words that are thought of as a single unit, but haven’t quite fused into a single word (the formal term for fusing into a single word is being “styled closed,” while a phrase written as two separate words is “styled open”). It’s always a good idea to check in whatever dictionary or style guide you use as your reference what the styling is for a particular compound noun. This is because there aren’t any specific rules. You just have to make a call and then be consistent (and always looking it up in the same dictionary or style guide will ensure you’re consistent). For example, “ink well,” “inkwell” and “ink-well” are all valid. If several valid options are giving in your dictionary or style guide, use the first.

Compound adjectives

Compound adjectives occur when you string together a number of words (not all of which are necessarily adjectives themselves) to describe a noun:

It was a once-in-a-lifetime opportunity.

The British rule is that you should always use the hyphens. The American rule is that you only need to use them when not using them would be ambiguous. My view is that if you use them all the time, then you don’t have to think about whether not using them would make your words ambiguous.

There’s an important difference between a compound adjective (where several words collectively form a single adjective) and a list of two or more adjectives, each of which modifies the noun independently, and hyphenation makes clear which of these cases applies.

For example, there’s a difference between a heavy-metal detector (which detects heavy metals) and a heavy metal detector (which detects metals, and is heavy). There’s a difference between a stolen property report (a report about a property, and the report has been stolen) and a stolen-property report (a report about some stolen property). The longest-known underground river in the US is the underground river that has been known about for the longest period of time, but the longest known underground river in the US is the known underground river that is the greatest in length (leaving open the possibility that there are longer rivers that we don’t know about). An article I read recently meant the latter but, because of over-hyphenation, said the former.

A common example of the compound-adjective pattern (see what I just did?) is this:

A two-year-old child.

Make sure you use both hyphens, as a two-year old child is, technically, a child that is both old and “two-year,” whatever that means.

English being English, you can drop out the well-understood noun and talk about a two-year-old. But if you’re saying “the child is two years old,” don’t use the hyphens, because then it isn’t a compound adjective–it’s not modifying a noun. It is, instead, modifying a verb (“is,” in this case), which makes it a compound adverb.

Compound adjective versus compound adverb

Compare these sentences:

The ever-faster cycles of the machine.

The machine cycled ever faster. 

In the first sentence, “ever-faster” modifies “cycles,” which is functioning as a noun. Therefore, it’s an adjective, and can take the hyphen.

In the second sentence, “ever faster” modifies “cycled,” which is functioning as a verb. Therefore, it’s an adverb, and does not take the hyphen.

For the same reason, don’t hyphenate phrases like this: 

I went to see her in person. 

Again, it’s an adverb (that’s how I went to see her) not an adjective, as it would be if I said: 

We had an in-person interview. (Compound adjective, modifying the noun “interview”.)

Also, don’t hyphenate compounds including “very” or other adverbs:

A very important meeting

A richly endowed foundation

(Good style advice is to avoid very in any case, and think of a stronger adjective in the first place: a vital meeting, for example. But sometimes very has its place.)

Note: Adding “-ly” to an adjective often transforms it into an adverb, but bear in mind that not every adverb ends in ‘-ly’ and not every word that ends in ‘-ly’ is an adverb. A word is an adverb if it modifies a verb, so “better,” “best,” “contrarywise,” “upward,” “tenfold,” “leftmost,” “not,” “downstage” and “anew” can all function as adverbs, while, for example, “only,” “likely,” “earthly,” “friendly” and “soldierly” are not adverbs but adjectives.

Non-hyphenated compounds

There are several examples in English of pairs of words that mean one thing when they are styled open and another thing when they are styled closed. The rules for some of these are similar to the hyphenation rules given above, but instead of having a hyphen, the adjective has become completely fused together as a single word.

She drank coffee every day. (Adverbial phrase, modifying “drank”.)

A visit to the café was an everyday ritual for her. (Compound adjective, modifying “ritual”.)

However, consider these special cases, which do not follow quite the same pattern. They all involve determiners, which are words that relate to quantity, such as any, some, or more.

Is there any way you can help me? (Determiner “any” modifying noun “way”.)

The cold never bothered me anyway. (Adverb “anyway” modifying verb “bothered”.)

I haven’t seen him in some time. (Determiner “some” modifying noun “time”.)

We should look in on him sometime. (Adverb “sometime” modifying verb phrase “look in”.)

I don’t want any more beer. (Determiner “any more” modifying noun “beer”; compare “I want some more beer.”)

I don’t want beer anymore. (Adverb “anymore” modifying verb “want”.)

Those last two examples are saying different things. “I don’t want any more beer” means “I have had enough beer for now”. But “I don’t want beer anymore” means “I used to want it, but now I don’t.”

In Britain, both usages of “any more” are frequently styled open, but in the US the adverbial version, referring to something that used to be true but is no longer, is styled closed (anymore).

It is incorrect in both dialects to style the non-adverbial version closed: “I don’t want anymore beer.”

Compound adjective versus adjective plus noun

Among the several ways I’ve seen writers over-hyphenate, one of the more common is adding hyphens to a phrase which is an adjective plus a noun, but is similar to a frequently used compound adjective.

For example: during the 19th century (adjective “19th” plus noun “century”) vs a 19th-century novelist (compound adjective “19th-century” modifying noun “novelist”); he was twenty years old (adjective “twenty” plus noun “years”) vs a twenty-year-old man (compound adjective “twenty-year-old” modifying noun “man”).  

Do not hyphenate a phrase in which the second part is functioning as a noun, rather than as part of a compound adjective.

Compound adjective vs verb plus preposition

There are a number of phrases that form one word or a hyphenated word when they are adjectives, but two when they are a verb-plus-preposition phrase, such as: stand out, set up, sign in. A preposition is a word that has to do with location (literal or figurative), such as out, up, in, on, under, from.

The Beatles stand out among bands of the 1960s for their popularity among their fans. (Verb and preposition.)

The Beatles gave a standout performance. (Adjective.)

The Beatles set up for their gig. (Verb and preposition.)

It’s quite a setup you have here. (Noun.)

We need an hour’s setup time to get it ready. (Adjective.)

Sign in with the app. (Verb and preposition.)

Please record your details on the sign-in sheet. (Adjective.)

During the Covid emergency, I have been repeatedly, though mildly, annoyed by the New Zealand government’s tracer app poster, which exhorts readers: “Sign-in. Stop the virus.” In this context, “Sign in” is a verb and a preposition functioning as a phrase; it’s not an adjective, and so it shouldn’t get the hyphen.  

Numbers

There’s one other place that I often see authors making mistakes with hyphens: numbers.

If you spell out the numbers twenty-one to ninety-nine inclusive, make sure to include the hyphens.

Do not, however, hyphenate hundreds (two hundred, not two-hundred) or thousands (two thousand, not two-thousand) when you spell them out. The same goes for millions, billions, etc.