Originally Posted by PairO'Dice Lost
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.
I know about the adjective thing, but it's the nouns and verbs that get me.
Why schoolwork but not school work or school-work? Why is well-being the preferred form, not wellbeing or well being? Why help desk but not helpdesk or help-desk?
Occasionally I can suss out some sort of reasoning between the three types of compound nouns, like the hyphen in re-elect making it clear that it's not pronounced "real ection". But then you get silliness like "preempt" which obviously doesn't follow the add-a-hyphen rule.
I wish English compounds worked like those in German, and we could just string them all together into one word and be done with it.
ETA: And I think the re-elect and preempt examples don't even count, since they're not proper compound words but rather words with prefixes. So my understanding of the reasoning is back to 0.