‘much-needed’ or ‘much needed’? When to hyphenate adverbs+participle/adjective

When you have an adverb ending in ly, followed by a participle or an adjective, do not hyphenate it, whether you are using it before or after a noun.

Example:

a highly paid banker

he was mildly amusing

BUT

For adverbs not ending in ly + participle or adjective, use a hyphen before a noun, but not after.

Example:

a much-needed addition

BUT

this addition was much needed.

the best-known author

BUT

He is best known for his research on migratory birds.

Compounds with more, most, less, least, very are generally open, unless ambiguity becomes an issue.

Example:

Most skilled workers had to also sit for a language proficiency test. [most in number, hence not hyphenated]

The most-skilled workers were recruited on a full-time basis. [most in skill, so hyphenated before noun (workers)]