Students who have trouble with fragments or run-on sentences, including comma splices, might want to study more about dependent and independent clauses.A clause is a group of words which work together and which contain at least a subject and a verb.

Main Clauses and Dependent Adverb Clauses

The following clauses are independent. They are complete sentences, or can be main clauses in longer sentences.

• I went to the skating rink yesterday.
• My paper was too long.
• Go home. (The subject is “you.”)

These clauses are dependent, or subordinate. They can not stand alone as complete sentences.

• After I went to the skating rink yesterday,…
• Because my paper was too long,…
• …if you go home.

A dependent clause must be paired with at least one independent clause to create a complete sentence.

• After I went to the skating rink yesterday, I ran into an old friend in the parking lot.
• You’ll miss the grand finale if you go home.

If a dependent clause appears before the main clause, there is a comma after the dependent clause. However, if the main clause is first, no comma is needed between them.

• Unless I hear from them soon, I’ll assume I didn’t get the job.
• I’ll assume I didn’t get the job unless I hear from them soon.

Words That Signal a Dependent Clause

These are words which writers commonly use at the beginning of a dependent clause.

• Time: after, before, until, when, while, as.
• Cause: because, since, so that.
• Condition: unless, although, if, as if, whether (…or not).

Other Types of Subordinate Clauses

Besides the adverb clauses described above, there are two other types of subordinate clauses, adjective clauses and noun clauses. The subordinate clauses in the sentences below are italicized.

• People who live in glass houses shouldn’t throw stones. (contains a relative clause which modifies “people”)
• That is the school where I attended eleventh and twelfth grades. (contains an adjective clause which modifies “school”)
• What you don’t know can hurt you. (contains a noun clause as the subject)

Remember that although a subordinate clause contains a verb, it cannot be the main clause. The following sentence is incomplete because it lacks a main clause.

• People who own exotic pets. (What about those people? The writer needs to complete the thought.)

Taken from: http://owl.ccd.edu/writ_resources/handouts/DepInd_Exp.html