A writer should hyphenate a compound modifier that

contains an adverb ending in - ly.

comes after the noun it modifies.

comes before the noun it modifies.

contains an adjective ending in - ive.

A writer should hyphenate a compound modifier that comes before the noun it modifies.

For example:

Can I get a twenty-dollar bill?

As you can see in this example, the compound modifier twenty dollar modifies the noun bill which is located after it, which is the reason why it has to be hyphenated.