Most but not all phrasal adjectives (two words that combine to modify a noun — hence the alternate name, compound modifiers) are hyphenated, which is confusing enough — though easily resolved: If a permanent compound is listed in the dictionary as open, no hyphen is necessary; otherwise, hyphenate. But that applies only before the noun.
What happens after the noun is a whole other matter: Usually, phrasal adjectives and similar (or similar-looking) constructions are left open in that position. Here’s a rundown on hyphenation rules for various types of compounds:
Age compound: “The eighteen-year-old (boy),” but “He is eighteen years old.”
Color compound: “The sky-blue paint,” but “The paint is sky blue.”
Fraction compound: “A half-mile walk,” but “a walk of a half mile.”
Number, spelled out: “Fifty-one,” “five hundred,” five hundred one,” “two thousand twenty-two.” (Hyphenate tens-ones figures in isolation and in larger figures, but leave open all other combinations of places.)
Number plus noun: “A five-year plan,” but a plan that will take five years”; “a four-and-a-half-inch gap,” but “a gap of four and a half inches”; “the fourth-floor office,” but “an office on the fourth floor.”
Number plus superlative: “The third-tallest player,” but “a player who is third tallest.”
Time: “They’re going to the eight o’clock screening” and “The meeting starts at six (o’clock)”; “I have a five-thirty plane to catch,” but “I’ll meet you at five thirty” (always open when time is on the hour, and hyphenated before the noun but open after when time is between hours).
Parts of Speech
Adjectival phrase: “His matter-of-fact manner,” but “His manner was matter of fact.”
Adjective plus noun: “A low-class joint,” but “The joint is low class.”
Adjective identifying origin or location plus noun: “An Indo-European language” and “the French-Spanish border,” but “She is a Japanese American” and “the latest Middle East crisis” (open unless the first term is a prefix or there is a sense of a distinction between the elements).
Adjective plus participle or adjective: “His long-suffering wife,” but “his wife is long suffering.”
Adverb ending in -ly plus participle or adjective: “Her rapidly beating heart” (always open).
Adverb not ending in -ly plus participle: “The little-read novel,” but “The novel is little read.” (See “More About Adverbs,” below.)
Noun phrase: “A feather in your cap,” but “He’s a jack-of-all-trades” (open unless hyphenated in the dictionary).
Noun plus adjective: “The family-friendly restaurant,” but “The restaurant is family friendly.”
Noun plus gerund: “A note-taking lesson,” but “a lesson in note taking.” (But beware of closed noun-plus-gerund compounds like matchmaking.)
Noun plus noun, the first one modifying the second: “A tenure-track position,” but “She’s on the tenure track.” (But leave permanent compounds like “income tax” open even before a noun, and check for closed noun-plus-noun compounds like bartender.)
Noun plus noun, equivalent: City-state, nurse-practitioner (always hyphenated).
Noun plus letter or number: “A size 34 waist,” “the type A personality” (never hyphenated).
Noun plus participle: “A problem-solving exercise,” but “time for some problem solving.”
Participle plus noun: “Working-class families,” but “members of the working class.”
Participle plus prepositional adverb plus noun: “Turned-up nose,” but “Her nose was turned up.”
More About Adverbs
When less or more modifies an adjective, such as in “a less frequent occurrence”/“an occurrence that is less frequent” or “a more qualified candidate”/“a candidate who is more qualified,” the phrase is not hyphenated either before or after a noun. The same is true of least and most unless ambiguity is possible.
For example, “a lesser-known rival” is a rival who is not as well known, but “a lesser known rival,” by contrast, might be a known rival of lesser consequence. Likewise, “the most-quoted orators” and “the most quoted orators” refer, respectively, to orators most frequently quoted and a majority of quoted orators. Again, however, the hyphenated version would be left open when it follows a noun, and would likely be worded differently than its counterpart that is not hyphenated before the noun, either.
Also, when an adverb that is part of a modifying phrase is modified by another adverb, as in “a very much praised debut,” the phrase is not hyphenated at all, even though a hyphen would appear in “a much-praised debut.”