Most punctuation is used to separate words or thoughts; not so with the hyphen and, to a lesser extent, the slash. They are used to join words where they might otherwise be either separate or “closed” (i.e., joined to form a single word).
The rules governing hyphens are like much of English grammar; they can serve a valuable verbal function while driving you to an altered state of sanity! There is hardly a rule regarding hyphens (and they are myriad) that does not have an exception. It is universal among the sources I have gone to for grammarians to say that in the end you (the writer) should or should not use the hyphen depending on how you feel about it (how’s that?).
This is not to diminish the hyphen; used properly, it serves a very important function and can materially alter the meaning of words by shifting emphasis. A simple phrase, a private school boy, will illustrate. These are words, easily understood out-of-context, but the phrase can change significantly by closing (joining) two of the words, as in schoolboy; or hyphenating two of them, private-school. If we are describing a quiet, shy person, we could say a private schoolboy; but if we merely want to describe the type of student he is (private vs. public school), we say a private-school boy. Same words, very different meaning because of punctuation; the hyphen in that case eliminates the ambiguity.
Of the many, fairly complex rules relating to hyphens the following seem to stand out:
(a) If an adverb precedes a verb or adjective and ends in ly, the hyphen should almost never be used. Some illustrations: newly married, mortally wounded, finely tuned, rapidly growing, fatally flawed, and internationally recognized.
(b) Adverbs that can also be adjectives and do not end in ly usually require a hyphen: well-dressed student, slow-moving traffic, ill-clothed man, and half-asleep audience. Note that all of these adverbs (well, slow, ill, and half) could be adjectives if they modified a noun; here though they are modifying verbs (dressed, moving, clothed, and asleep) and become adverbs. On the other hand, words that are always adverbs, such as too, very, almost, seldom, and not are not hyphenated.
(c) If a compound adjective precedes a noun, it should be hyphenated: well-known actress, bone-numbing chill, American-style music, 14-year-old student, and entry-level position. This is a list that could go on indefinitely; note, though, by altering the position of the modified noun, as in the actress is well known (actress now precedes the compound adjective), we cause the hyphen to disappear. Note further that phrases which are compound adjectives often consist of non-adjectives; in the example cited bone, style, 14, year, entry, and level are all nouns, but when coupled with other words they become adjectival phrases.
(d) Numbers beyond twenty and not divisible by ten, such as twenty-two, thirty-five, seventy-seven, and two hundred fifty-four, should have hyphens as should fractions: two-thirds, one-half, one-quarter. A compound adjective containing a number immediately followed by a noun should be hyphenated: three-yard rope, twenty-mile limit, twenty-two-day vacation.
What are we to make of all of this? We are told that there is a tendency to join or close words (schoolboy is one illustration) which were formerly hyphened. In many cases, if you are not absolutely clear, consult your dictionaries, though let me warn you: there is anything but universal agreement among them. Apparently our British cousins are far more likely to hyphenate than are American writers (as Churchill said “One people, divided by a common language”); they will write living-room, dog-dish, and sitting-room (none of which is hyphenated here). British dictionaries reflect this when compared to their American counterparts.
So remember, use the hyphen if it feels right but use it sparingly. Coupled with that, however, is the admonition to be consistent. And, perhaps most important, if you have the slightest doubt, pull that Merriam-Webster off the shelf.
— Ken Butera