A modifier is usually an adjective, but can be any word that helps to shape (‘modify’) our opinion of the subject of a sentence. Since a modifier has to impart more information about something, by definition that means the something it is modifying or limiting has to exist. That means, of course, that you can’t just say The happy. If you did, people would immediately ask you: “the happy what?” That missing what is the thing being modified.
It seems pretty obvious and intuitive when written in a simple sentence, and it seems hard to imagine a situation in which a modifier would be left dangling. However, modifiers don’t always have to be simple words or phrases like happy, and sentences aren’t always simple.
Phrases can also act as modifiers, providing additional information about something else in the sentence. When this occurs, and when sentences become more complex, dangling modifiers can sometimes exist and get lost in the complexity of the language.
http://examples.yourdictionary.com/examples-of-dangling-modifiers.html
Here is a classic example:
