Hyphens are used to link words and parts of words. They are not as common today as they used to be, but there are three main cases where you should use them:
Hyphens are used in many compound words to show that the component words have a combined meaning (e.g., pick-me-up, mother-in-law, good-hearted) or that there is a relationship between the words that make up the compound: for example, rock-forming minerals are minerals that form rocks. But you don’t need to use them in every type of compound word.
Compound adjectives are made up of a noun + an adjective, a noun + a participle, or an adjective + a participle. Many compound adjectives should be hyphenated. Here are some examples:
noun + adjective
noun + participle
With compound adjectives formed from the adverb well and a participle (e.g., well-known), or from a phrase (e.g., up-to-date), you should use a hyphen (or hyphens) when the compound comes before the noun:
well-known brands of coffee
an up-to-date account
but not when the compound comes after the noun:
His music was also well known in England.
Their figures are up to date.
It’s important to use hyphens in compound adjectives describing ages and lengths of time: leaving them out can make the meaning ambiguous. For example, 250-year-old trees clearly refers to trees that are 250 years old, while 250 year old trees could equally refer to 250 trees that are all one year old.
Use a hyphen when a compound formed from two nouns is made into a verb, for example:
an ice skate
a booby trap
a spot check
a short circuit
You should NOT put a hyphen within phrasal verbs, which are verbs made up of a main verb and an adverb or preposition. For example:
The government tried to cover up the incident.
They broke in by forcing a lock on the door.
We stopped off in Hawaii on the way home.
If a phrasal verb is made into a noun, though, you SHOULD use a hyphen:
She denied that there had been a cover-up.
The house was unoccupied at the time of the break-in.
We knew there would be a stop-off at Singapore for refueling.
A compound noun is one consisting of two component words. In principle, such nouns can be written in one of three different ways:
In the past, these sorts of compounds were usually hyphenated, but the situation has changed. The tendency is now to write them as either one word or two separate words: the hyphenated forms are generally the least common. However, the most important thing to note is that you should choose one style and stick to it within a piece of writing. Don’t refer to an chatroom in one paragraph and a chat room in another.
Hyphens can be used to join a prefix to another word, especially if the prefix ends in a vowel and the other word also begins with one (e.g., pre-election or co-own). This use is less common than it used to be, though, and one-word forms are becoming more usual (e.g., prearrange or cooperate).
Use a hyphen to separate a prefix from a name or date. For example: post-Aristotelian or pre-1900.
Use a hyphen to avoid confusion with another word. For example, to distinguish re-cover (= provide something with a new cover) from recover (= get well again).
Hyphens can also be used to divide words that are not usually hyphenated.
They show where a word is to be divided at the end of a line of writing. Always try to split the word in a sensible place, so that the first part does not mislead the reader. For example: hel-met not he-lmet; dis-abled not disa-bled.
Hyphens are also used to stand for a common second element in all but the last word of a list. For example:
You may see a yield that is two-, three-, or fourfold.