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The two clause types differ by whether they can stand alone. An independent clause is a complete sentence with a subject and a verb; a dependent clause leaves the reader waiting for more, so it cannot stand on its own.
The most common signal of a dependent clause is a subordinating conjunction at the start — words like "because," "as," "when," "although," "while," "since," and "if." Take the full sentence "As the diver descended into the abyss, she felt a sense of exhilaration." It splits into two clauses: "As the diver descended into the abyss" is dependent, because it begins with "as" and cannot stand alone, while "she felt a sense of exhilaration" is independent, with the subject "she" and the verb "felt."
A clause is also dependent when it lacks a full subject-and-verb pair. In the full sentence "Condemning most of his later work, Tolstoy singled out Chekhov for praise," the opening "Condemning most of his later work" has no subject and verb of its own, so it is dependent, while "Tolstoy singled out Chekhov for praise" is independent, with the subject "Tolstoy" and the verb "singled out."
Telling the two apart matters for punctuation. A dependent clause joined to an independent one usually takes a comma, not a period or semicolon — those separate two independent clauses, which is why identifying the clause type is the first step in boundary questions.
Worked examples
Is "when they first faced off in a competitive match" independent or dependent? Dependent. It begins with the subordinating conjunction "when" and cannot stand alone as a complete sentence.
Is "Condemning most of his later work as bad" independent or dependent? Dependent. It has no complete subject-and-verb pair forming a full thought, so it cannot stand alone.
In "Tensions between the two nations escalated as each sought to outdo the other," which part is dependent? "as each sought to outdo the other" is dependent — "as" subordinates it — while "tensions between the two nations escalated" is the independent clause (subject "tensions," verb "escalated").
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