My belief is that the following two phrases are correct:
A: "The app is easy to use." B: "It is an easy-to-use app."
And that the following is not technically correct:
C: "It is an easy to use app."
If anyone can point out that C is correct versus B, would you mind pointing to an English language "rule" that shows why? My general rule-of-thumb is that if a section of the compound adjective cannot exist on its' own merit, hyphenate.
Thank you.
Prepositive modifiers don't like to have postpositive dependents. The more common pattern employs prepositive dependents:
It is a very easy app.
The "very" is a prepositive modifier of "easy", and the phrase "very easy" is prepositive to the "app" that it modifies.
Another common pattern has postpositive dependents for a postpositive modifier:
The app is easy to use.
When a prepositive modifier has a postpositive dependent, there is a conflict. The prepositive modifier is expected to modify the next word, but there's a postpositive dependent competing for that same word position.
The way to solve the word placement competition is to treat the phrase "easy to use" as a single word. The hyphens join the more tightly bound phrase. Inside the hyphenated phrase, the expectation of the postpostive dependent position of "to use" is fulfilled. As a result, the "I'm modifying the very next word" expectation of the prepositive "easy-to-use" is easy to fulfill.
This is an easy-to-use app.
Postpositive modification inside the hyphenation, prepositive modification outside. The shift between right-to-left parsing and left-to-right parsing is clearly marked.
Edit:
There is another word ordering that might be worth comparative consideration:
This is an easy app to use.
In this example, "to use" is definitely a postpositive dependent, but the word from which it depends is ambiguous. In this position, "to use" could modify either "app" or "easy".
This ambiguity does not result in any word-ordering conflicts. Regardless of whether "to use" depends from "easy" or from "app", it is in its expected postpositive position. The boundary between left-to-right and right-to-left parsing does not need to be marked.
You can check here:
A compound adjective usually gets a hyphen when it comes before a noun, like in ‘a well-honed piece of writing’.
But if the first word ends in ‘ly’, like in ‘a specially designed workshop’, you don’t need a hyphen.
When it comes after the noun, the compound adjective usually doesn’t get a hyphen. So we say an easy-to-remember number, but the number is easy to remember. Same goes for up to date – if it’s before a noun it needs a hyphen. A document is up to date but it’s an up-to-date document.
So you are right in saying that statement B is correct and C is incorrect. Since the noun 'app' is preceded by the compound adjective, it is to be hyphenated as done in statement B.
