Why compound modifiers are not usually hyphenated when they come after the noun?

Question

A lot of grammar references suggest that you hyphenate compound modifiers if they come before the noun but not after the noun. Here's an example:

1. The apartment is off campus.

VS

2. That is an off-campus apartment.

One of the references that I saw, https://www.quickanddirtytips.com/education/grammar/are-you-using-hyphens-correctly, states this:

Ages are like every other compound modifier that way: you hyphenate them before the noun but not after the noun.

So it says that compound modifiers are not hyphenated when they come after the noun.

I have also read many other English references that say that. Why is it the case? Aside from it is good and it is recommended, is there a technical reason or it is a matter of style?

Thanks!

Answer 1

Hyphenation of modifiers before the noun to be modified is necessary to prevent ambiguity or misinterpretation.

The type of "apartment" in this sentence is one that is not located on the campus. As such, "off" modifies "campus", not "apartment".

If one modifier is modified by another word, in this case "off", then it must be connected to it's noun, in this case "campus", to define the specific meaning of the modifier before the main noun. Otherwise, disconnected compound modifiers may confuse meaning.

If the multiple modifiers are all specific to the main noun on their own, then commas would be used to separate them.

"...clean, safe, off-campus apartment"

When the compound modifier comes after the noun, the hyphenation is unnecessary to show the two go together. In your example, "off campus" is the predicate adjective following the linking verb "is".