Someone sent me a link to a blog post about hyphenated compound adjectives in which the rules were completely mangled because the author didn't distinguish between adjectives and adverbs. (I'm not going to link to that original post because my purpose is not to embarrass or harass that author, but to state the correct rule.) This is one of those things that causes a crapton of confusion, so I thought it would make sense to review the rule. (Related posts: how to identify compound adjectives, squinting modifiers)
First, you have to know the difference between an adjective and an adverb.
Adjectives modify nouns in a way that describes an attribute of the noun.
Adverbs modify adjectives, verbs, or other adverbs in a way that describes a relation or degree of time, degree, manner, and similar qualities.
(inflated is an attribute of the noun balloon, so inflated is an adjective -- technically, a past participle functioning as an adjective)
partially inflated balloon
(partially is a degree of inflated, so partially is an adverb modifying an adjective)
If you were to hyphenate that,
you would be wrong.
This is considered an "exception" to the rule regarding hyphenation of compound adjectives, but that's something of a misnomer. This could never be a compound adjective because it's half-adjective, half-adverb. A similar "exception" involves whether to hyphenate a preposition used as an adverb, such as:
Please check out at the check-out counter.
The first "out" is an adverb modifying the verb "check." The second "out" is part of a compound adjective modifying "counter."
One place where the rule is in flux regards the use of comparative or superlative compounds. The old rule is that you never hyphenate a comparative or superlative adverb modifying an adjective. This is probably easiest to see if we stick to good/well, better, best as our example because that will ignore the -ly comparatives and superlatives.
better built car
best dressed woman
well read man
a good, simple dinner (good is an adjective modifying a noun)
a well prepared dinner (well is an adverb modifying an adjective)
the best cooked dinner (best is a superlative modifying an adjectival past participle)
That's the old rule. Lately, we've seen people hyphenating these usages, but to my eye, it looks strange. Maybe this is because my eye knows the difference between adjectives and adverbs, and my eye knows that you don't hyphenate an adverb to an adjective, but whatever. Rules change.
What are compound adjectives? When two adjectives operate together as a unit to modify a noun. For example,
Solution is the noun. Long and term operate together to create a single unit of meaning. This isn't an arbitrary rule because it can have an impact on meaning. Compare:
The large appliance factory is closing.
The large-appliance factory is closing.
In the first, an appliance factory which is large is closing.
In the second, a factory which makes large appliances is closing.
Okay, so, that probably doesn't un-confuse anybody. As I said, this is not an easy concept to grasp, and even seasoned copy editors sometimes quibble over particulars, especially in the case of squinting modifiers (see link at the top of this post). But I thought it was worth at least trying to un-confuse things. :)