What is a sentence fragment?

Sentence fragments are groups of words that look like sentences, but aren't. To be a sentence, groups of words need to have at least one independent clause.

An independent clause is any group of words that contains both a subject and a verb and can stand on its own. For example, 'I like cheeseburgers' is an independent clause.

Sentence fragments never have independent clauses, but instead are dependent clauses or phrases.

Sentence fragments usually appear before or after the independent clauses to which they belong. For example:

'When we got in the car. We rolled down the windows.'

'When we got in the car' is a sentence fragment and a dependent clause. It clearly belongs to the independent clause that follows it and should be rewritten like this:

'When we got in the car, we rolled down the windows.'

Or like this:

'We rolled down the windows when we got in the car.'