Understanding the difference between independent and dependent clauses is important in understanding how to construct sentences and avoid fragments. This knowledge can also help in varying sentence length in writing, which makes all forms of writing better (More on sentence length).
Before understanding the different types of clauses, it is important to understand what a clause is. A clause is a group of related words that contains both a subject and a verb. On the other hand, if a group of related words does not contain a subject with an attached verb, it is simply a phrase.
An independent clause is a group of words that can stand on its own as a sentence: it has a subject, a verb, and is a complete thought.
A dependent clause is a group of words that also contains a subject and a verb, but it is not a complete thought. Because it is not a complete thought, a dependent clause cannot stand on its own as a sentence; it is dependent on being attached to an independent clause to form a sentence.
Dependent clauses can often be identified by words called dependent markers, which are usually subordinating conjunctions. If a clause begins with one of these words, it is dependent and needs to be attached to an independent clause. (Common dependent markers: after, as, although, because, before, even though, if, once, rather than, since, that, though, unless, until, when, whenever, whereas, while, among others.)
Note: Just because an independent clause can stand on its own doesn’t mean it has to. For instance, one or more independent clauses can be added together to form a compound sentence, and independent clauses can be added to dependent clauses to form complex sentences. (More on the different types of sentences)