When we construct a compound modifier with a noun, an adverb that doesn’t end in -ly, or an adjective and a present participle (the –ing form of a verb) before a word to modify it, we need to use a hyphen in our compound modifier.
Example:
1) We often use fast-acting medications in emergencies.
2) The medications we often use in emergencies are fast acting.
Note that in the first sentence (1), fast-acting is used before medications and therefore has a hyphen, but in the second sentence (2) fast acting doesn't have a hyphen because it comes after medications, which is the noun it describes.
When we construct a compound modifier with a noun, an adverb that doesn’t end in -ly, or an adjective and a past participle before a word to modify it, we need to use a hyphen in our compound modifier.
Example:
1) The well-known writer signs her books at the library tonight.
2) The writer who signs her books at the library tonight is well known.
Note that in the first sentence (1), well-known is used before writer and therefore has a hyphen, but in the second sentence (2) well known doesn't have a hyphen because it comes after writer, which is the noun it describes.
