hyphenation - "easy to use" versus "easy-to-use"

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.

Answer

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.

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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.