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 …Read more >