Have I really not written about this? Wanted to find a link to a post explaining hyphenation for a client, and couldn’t find it. Ohhh well: let’s start here.
Hyphenating is a way of stringing words together to make a single expression, creating new meaning or enhancing the member words’ meaning by showing the relationship between the words. This is handy-dandy (handy and dandy! hot diggety!), but there are a few rules to standardize this trick.
Let’s say you have a leaky pipe. It’s eight inches long. To describe it to a plumber over the phone, you would say…
“It’s an eight-inch pipe.”
Here, you’ve run an adjective (eight) together with a noun (inch) to create a combination that you can use adjectivally, to describe a noun (pipe). I like to call these hyphenated combos “word strings.”
“Huh?” says the plumber, who’s a bit hard of hearing. “Come again?”
“It’s an eight-inch-long pipe.”
In both cases you’ve run together several words to modify the word pipe. Now, to try to get your point across, you’ve added another adjective (long) to your word string. But, says the plumber, “I just can’t picture that.”
“It’s eight inches long.”
Each of these words stands on its own. How long is it? Inches. Oh yeah? How many inches? Eight.
So: hyphenate words that you need to join together to make them function as a unit, adjectivally.
Now, lissen up:
A kid that is eight years old is…what? Yes:
She is an eight-year-old.
But if someone asks you how old she is and you have to put your answer in writing?
She is eight years old.
When you run 8 + year + old together to create a noun denoting the type of creature the kid is, then you hyphenate. But when you do not use the run the words together to make a noun — She is _____ old; she is ___ years old; she is eight years old — then you don’t hyphenate. Because old is an adjective describing the kid; years is used as adjective to modify old, and eight is an adjective modifying years. “Eight years old” is NOT a word string; it’s just three words used in a sentence.
Thus “she is eight-years-old” is what we call a monstrosity.