1 - flesh-eating animal. Eating is not a verb in this clause, it is a noun. The gerund form of a verb (-ing form) is used as a noun to describe the act of performing whatever the verb is (in this case the act of eating). Whenever you hyphenate two nouns, the first is acting as an adjective on the second. "What kind of animal? An eating animal. What does the animal eat? It eats flesh. It is flesh-eating. It eats apples. It is apple-eating. etc". 2 & 3 - These are different because of the words you chose. "Heartbroken" and "heartbreaking" are non-hyphenated English words. Example two should therefore be "A heartbreaking book.". Example three is a bit different. "A heartbroken girl" is a girl whose own heart has been broken by something or someone else. "A heartbreaking girl" is a girl who breaks the hearts of other people! English can be strange that way!