Hyphenate an adverb-adjective pair when the adverb does not end with -ly and and the pair is modifying a noun directly.
For a single sentence, there are a number of places that this can go horribly wrong. For one thing, many of identify adverbs by looking for that little -ly tagged onto the end of a familiar adjective. "Easy" becomes "easily," "beautiful" transmutes into "beautifully," and "definite" firms up into "definitely."
My sister is a well-known actress. Her name is June Bug.
She can't be that well known. I've never heard of her.
In the first sentence, "well-known" is directly modifying the noun "actress." Meanwhile, in the reply, "well known" is acting as the object of the clause, thus there is no hyphen. The value of these hyphens is in clarity. By hyphenating "well-known", it's making it clear that the two words are working as a unit to modify the word "actress." Without the hyphens, though, and it becomes "well known actress". "Well" modifies "known", which modifies "actress". But what is the relationship between "well" and "actress"? It's unclear. The rule becomes clearer with a hyphenated adjective phrase with more than two words.
You have a real holier-than-thou attitude, you know that?
Maybe I am holier than thou! Did you ever think about that?
The above example starts to demonstrate the real power of the hyphens. In two-word combinations, the meaning doesn't get quite so jumbled. In the case of three-word combinations. The preceding example also shows an example of words besides just adverbs and adjectives working together. In "holier-than-thou", there is a comparative adjective, a conjunction, and a pronoun. In the following example, it's adjectives and nouns.
I have a three-year-old daughter.
My son is three years old, too! What a coincidence!
Beside the fact that it isn't really enough of a coincidence to make it worth mentioning, this is one of the most banal examples of the hyphenated words modifying a nouns. I remember this particular example from a job interview for an editing position. I missed this particular mistake - I didn't even realize it was a mistake that the time - and missed the job as a result. It wasn't the only mistake I missed, but there is a real-world consequence of a grammatical mistake. What can I say? It was early in my literary career, but I will never forget this particular rule. Not on purpose, at least.