What is a dangling modifier?

First off, let's define a modifier: a word, phrase, or clause that gives you more information about a subject or concept. Modifiers need to be placed directly before or after your intended target. Here's an example:

Forgetting about the exam, Crystal rushed to make it to class before the bell rang.

The modifier forgetting about the exam gives the reader more information about Crystal and her state of mind.

When your modifier doesn't come right before or after your subject, it dangles hopelessly out of place, changing the meaning of your sentence.

Considering going to bed early, the sun set behind the boy's tired eyes.

Well, is the sun considering going to bed early in this sentence or is it the boy? The dangling modifier is hung out there with no hopes of creating an intelligent sentence.

When you use a phrase or clause, identify the main clause of your sentence and make sure your modifiers come either directly before or after your subject.