The simple answer is that an independent clause is a sentence and a dependent clause isn’t. However, groups of words can be dependent clauses or phrases. A prepositional phrase, for example, does not contain a subject and a verb. But a dependent clause does.
An independent clause needs a subject and verb, and a dependent clause also needs both. So what is the difference between the two? Simple.
An independent clause can stand alone as a sentence. A dependent clause cannot–it needs to be connected to an independent clause; otherwise, it is a sentence fragment.
What is a Sentence Fragment?
A FRAGMENT is a group of words that are not a complete sentence. We know that “in the morning” is not a sentence, and “loves to swim” is not a sentence either. Instead, “in the morning” is a prepositional phrase and “loves to swim” can function as the predicate.
In the context of the difference between dependent and independent clauses, a sentence fragment is a group of words that include a subject and a verb but cannot stand alone, and an independent clause is a self-contained sentence.
What Makes a Dependent Clause Not Independent?
A dependent clause is a group of words that have a subject, verb and a type of conjunction that turns what could be a sentence into a dependent clause.
It would be easy if this type of conjunction were called a dependent conjunction, or a dependisizer. However, it is a subordinating conjunction, which we talk about in detail in this post.
If you need to determine if a word is a subordinating conjunction, place the word in front of a sentence. A subordinating conjunction turns the sentence into a fragment. Or in English teacher speak, the subordinating conjunction turns an independent clause into a dependent clause.
To test if a word (such as after) is a subordinating conjunction, test it with a short, memorable sentence. Make your test sentence memorable:
The boy ran after his pet turtle.
The above sentence is an independent clause.
When the boy ran after his pet turtle.
The addition of the subordinating conjunction turns the sentence into a dependent clause. Unless it is attached to an independent clause or the subordinating conjunction is replaced by a conjunctive adverb (however, therefore, and consequently are three common conjunctive adverbs).
Adding a conjunctive adverb in front of the memorable sentence (or independent clause) does not turn it into a dependent clause.
Therefore, the boy ran after his pet turtle.
A conjunctive adverb suggests that something happened before the kid ran after his turtle. However, the subordinating conjunction implies that something else will happen.
The difference between dependent and independent clauses is that an independent clause is a sentence and a dependent clause is a fragment. Both have a subject and verb, but the dependent clauses also has a subordinating conjunction in front of it.
And if you want to take a deeper dive into prepositions and other myths of English grammar, check out this Smithsonian magazine article.