Hyphens are used in compound adjectives before nouns (a well-known author, a thirty-year-old person) and compound numbers from twenty-one to ninety-nine (forty-five, ninety-nine). Some compound nouns are hyphenated (mother-in-law, merry-go-round), while others have become single words (baseball, textbook) or two words (ice cream). Different style guides have slightly different rules, but the goal is always to clarify meaning and improve readability.
The hyphen's core job is to prevent misreading. When two or more words work together as a single modifier before a noun, a hyphen signals that they form a unit. Consider "a small business owner" versus "a small-business owner." Without the hyphen, a reader might parse "small" as modifying "owner" (a petite person who owns a business). With the hyphen, "small-business" clearly forms a single compound adjective modifying "owner." This compound modifier (also called a compound adjective) rule is the most consistently applied hyphen rule in modern writing: if two or more words function as a joint adjective *before* the noun they modify, hyphenate them.
The position of the compound matters. When the same modifier comes *after* the noun, the hyphen is usually dropped: "the owner is well known" (no hyphen) but "a well-known owner" (hyphen). After a linking verb, the modifier is in predicate position and the hyphen becomes unnecessary because there's no ambiguity about what modifies what. This is why "a well-written essay" takes a hyphen but "the essay was well written" does not.
Compound numbers from twenty-one to ninety-nine always take hyphens when written out, regardless of their grammatical role. This is one of the few mechanical hyphen rules without exceptions: forty-five, sixty-three, ninety-nine. Fractions used as adjectives are also hyphenated (a two-thirds majority), though this varies by style guide. Compound nouns are trickier because they exist along a spectrum of consolidation: new compounds start as two separate words (web site), gain a hyphen as they become established (web-site), and often merge into a single word over time (website). Whether a compound noun is hyphenated, solid, or open today depends on usage frequency and your style guide — when in doubt, check a current dictionary.
The underlying principle in all these cases is the same you learned with compound nouns: when words fuse into a single concept, spelling reflects that fusion. Hyphens are the intermediate stage — the orthographic signal that these words are working as a team but haven't yet merged completely. A reader who sees a hyphen should think: these pieces belong together.