A dependent clause is a group of words that has a subject and a verb, but it cannot stand alone as a complete sentence. It's like a baby or a young child; it has everything it needs to be a person, but it can't survive on its own. It needs to be attached to an adult—a complete sentence—to make sense.
The reason a dependent clause can't stand alone is that it begins with a special word, called a subordinating conjunction or a relative pronoun, that makes it an incomplete thought. It leaves the reader hanging, waiting for more information.
1. The Clause "Because the dog was happy"
Problem: If you just say, "Because the dog was happy," it leaves you asking, "What happened because the dog was happy?" It's an incomplete thought.
Correction: You must connect it to a main idea.
The boy threw the ball because the dog was happy.
We went to the park because the dog was happy.
2. The Clause "After the movie ended"
Problem: If you only write, "After the movie ended," the reader wonders, "What happened after the movie ended?"
Correction: You need to attach it to a complete sentence.
After the movie ended, we went out for dinner.
I checked my phone after the movie ended.
Look out for these words at the beginning of a clause. They are often clues that the clause is dependent and needs to be connected to a main idea.
Subordinating Conjunctions: after, although, as, because, before, if, since, unless, until, when, while.
Relative Pronouns: who, whom, whose, which, that.
The most important thing to remember is that a dependent clause is a fragment when it's by itself. It needs to be joined to a complete sentence.