Whether something is one word, two words, or hyphenated, often drives writers crazy. Is it goodbye or good-bye? Fairy tale? Fairytale? Or fairy-tale?
A quick way to check is to go to onelook.com.
It has 18 hits for good-bye and 23 for goodbye. A good rule of thumb is to go with the majority, so in this case, that’s goodbye. However, Merriam Webster is one of the 18 for good-bye and since many companies use Merriam Webster as their dictionary of choice, here, good-bye is an equally good pick.
Fairy tale is even worse. There are 11 hits on fairy tale, 8 on fairy-tale, and 17 on fairytale. And in this case Merriam Webster has it hyphenated, so any of the three is a reasonable choice.
My advice would be 1. Stick with the majority. Or 2. Stick with Merriam Webster. That is, unless you are targeting your book for a certain publisher. In that case always follow their style guide.
Do I hear you asking, “What about compound adjectives? Why is it a red-hot fire, but a brightly lit room?”
Now that one is easy. Words ending in ly are (usually) adverbs. By definition an adverb can never be a compound adjective.
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