Generally, hyphens should be used with compound words (nouns, verbs, adjectives) or as part of a prefix. There are other times to use them, but these are the most common.
Compound nouns, such as 'father-in-law', use hyphens to show that each part has a distinct meaning. So, we hyphenate 'father-in-law' to show the meaning, not to mention that 'fatherinlaw' looks ridiculous. 'Switchblade', while a compound noun, would not be hyphenated, because we gain no additional definition from 'switch-blade'.
Compound verbs appear when the verb is formed from two nouns. "The Mad Trapper set a booby trap. He booby-trapped the computer."
(Yes, booby trap can be hyphenated when used as a noun as well. It gains no additional meaning by doing so, but it makes some people feel better about using the word 'booby' in a sentence.)
Compound adjectives are combinations of nouns, adjectives, or participles, and are hyphenated to indicate that they are one word, and for clarity. Someone is hot-blooded, because their blood is hot, and they are prone to temper. Using 'hot blooded' instead is confusing, because the two separate words indicate that they are both hot and have recently inflicted injury on someone.
Another example of when to use a hyphen for a compound adjective: "200-year-old ghouls" indicates any ghouls who are 200 years old. "200 year old ghouls" could mean a group of 200 ghouls who are all one-year-old.
Prefixes are hyphenated in three cases:
- When the prefix ends in a vowel, and the word begins with a vowel (e.g., co-opt, re-enter).
- To avoid confusion with another word (e.g., re-cover when you replace a cover, and recover when you get better).
- To separate the prefix from a name or a date (e.g., pre-War).
As a general rule, if it's not one of the above situations, and you just used a hyphen... well, re-read it and reconsider. A common habit that people get into is using hyphens instead of periods or commas. This leads to the monstrous run-on sentence, and unless it's proofed or re-read, ends up getting published and looking bad.
Scorpitron 21:18, December 28, 2011 (UTC)