Hyphens are much less common today than they used to be. In most instances you can probably get away with omitting them entirely. Here are just a few exceptions.
1. Hyphens are used in many compound words to show that the component words have a combined meaning (e.g. a 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.
2. 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 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.
3. 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.
4. Use a hyphen to separate a prefix from a name or date, e.g. post-Aristotelian or pre-1900.
5. 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).
6. 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.
7. Hyphens are also used to stand for a common second element in all but the last word of a list, e.g.: You may see a yield that is two-, three-, or fourfold.
8. Link compound proper nouns and adjectives with a hyphen. For example, "Italian-American."
9. Use a hyphen with numbers and fractions. For example, "four-fifths," "thirty-eight" and so on.