Originally Posted by historiasdeosos
I hate hate hate English compound words, because there seems to be no rhyme or reason behind which are joined together, which are hyphenated, and which are separated and just work together as a "phrase".
In general, hyphenated words are adjectives and non-hyphenated or compound words are nouns. The difference between a hyphenated word and a non-hyphenated phrase is that hyphenated words are those in which their constituent parts can't act individually while phrases do have constituent parts that all act individually.
Hyphenated words such as in, say, "man-eating shark," are compound modifiers, two words that, when used in conjunction, modify their object in a way different from the way they would individually; the shark isn't a man shark, and it isn't an eating shark, it's a shark that eats men and thus a man-eating shark. An egg beater, by contrast, is a thing that beats eggs, and so doesn't need a hyphen because both words are using their individual meanings.