Use a hyphen when two or more words come together as a single adjective before the noun.
INCORRECT: The boys enjoyed making fire roasted marshmallows.
CORRECT: The boys enjoyed making fire-roasted marshmallows.
Use a hyphen when you need to form a verb from a pair of nouns.
INCORRECT: After the campfire, the boys planned to ice skate.
CORRECT: After the campfire, the boys planned to ice-skate.
If you need to spell out a compound number, use a hyphen. This type of usage often occurs at the beginning of a sentence, since it is better style to write compound numbers in the middle of the sentence as numerals instead of spelling them.
INCORRECT: Twenty six children went on the camping trip, but only two boys roasted marshmallows at the campfire.
CORRECT: Twenty-six children went on the camping trip, but only two boys roasted marshmallows at the campfire.
Hyphens should always accompany certain prefixes, including ‘all-,’ ‘anti-,’ ‘mid-,’ and ‘ex-’ (when it means the same thing as ‘former’), even if the prefix modifies a capitalized word or a number.
INCORRECT: Back in the mid 1990s, two self-assured all American boys roasted marshmallows over a fire.
CORRECT: Back in the mid-1990s, two self-assured all-American boys roasted marshmallows over a fire.
Sometimes, adding a prefix to a word makes that word look like a word with a totally different meaning. (Grammarians use the word ‘homograph’ for two words spelled the same but with different meanings.) To avoid this sort of confusion, you may choose to add a hyphen between the prefix and the word.
INCORRECT: He attempted to recover the marshmallow from the embers so he could recover it in graham crackers.
CORRECT: He attempted to recover the marshmallow from the embers so he could re-cover it in graham crackers.
If the rules listed above do not suffice for a word (or combination of words) you plan to use, use a dictionary to look it up and see if the use of a hyphen is recommended, or whether the two words are joined together or separated with a space. If you cannot find the compound word in the dictionary, use a space between the two words.
INCORRECT: ‘You can become an everyday astrophysicist,’ he replied, ‘but I want to go on camp outs as a scout master.’
CORRECT: ‘You can become an everyday astrophysicist,’ he replied, but I want to go on camp-outs as a scoutmaster.’