A compound modifier is two or more words working together as a single description before a noun — "well-known," "five-year-old," "state-of-the-art." Whether to hyphenate them follows one consistent rule, even though it doesn't always feel that way.
Hyphenate two or more words when they combine to modify a noun that comes right after them. Don't hyphenate the same words when they appear after the noun instead.
Before a noun, unhyphenated words can genuinely be misread as separate modifiers rather than one combined idea. "A well known actor" reads for a split second like "well" and "known" are two independent descriptions rather than one phrase meaning "famous." The hyphen locks them together as a single unit so the reader doesn't have to pause and reparse.
Don't hyphenate when the first word is an adverb ending in -ly, even before the noun — the -ly ending already makes it unambiguous that it's modifying the next word, so the hyphen is redundant.
The same before/after rule applies to number-plus-noun modifiers:
Ask: is this phrase sitting directly in front of the noun it describes? If yes, and it's not an -ly adverb, hyphenate. If the same words appear anywhere else in the sentence, leave the hyphen out.
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