A clause contains a subject -- who or what the sentence is about -- and a predicate -- the verb and sometimes other words that modify the subject. As a group of words, a clause can compose part of a sentence or a whole sentence. Understanding the differences in the two main types of clauses, subordinate and insubordinate, can help you determine whether to use a semicolon or a comma and whether you have a complete sentence or a fragment.
Every sentence contains at least one clause; some have multiple clauses. A string of words without clear meaning is not a sentence, but a fragment. There are two main types of clauses: insubordinate clauses, more commonly referred to as independent clauses, and subordinate clauses, also called dependent clauses.
An insubordinate clause consists of a subject and a verb and conveys a complete thought. For example, the insubordinate clause "the sky is blue," contains a subject (sky) and a verb (to be) and conveys a complete thought. An insubordinate clause forms a simple sentence. It can stand alone - -independent from other clauses -- and still make sense.
A subordinate clause consists of a subject and a verb but does not convey a complete thought. It usually begins with a dependent word or phrase such as after, as if, because, even though, unless, whatever, when or while. For example, the subordinate clause "while Susie went to the store..."contains a subject and a verb, but it does not convey a complete thought. This fragment depends on an insubordinate clause to complete the sentence.
Separating part of a sentence to see if it makes sense alone is a good way to identify clauses. For example, the sentence "While my parents were away, I had a party at our house" can be separated into two parts. The first part, "While my parents were away," does not make sense on its own. It depends on the second part of the sentence for meaning, making it a subordinate clause. The rest of the sentence, "I had a party at our house," is an insubordinate clause and does make sense alone.
Commas and Semicolons
Understanding the difference between subordinate and insubordinate clauses makes it easier to decide when to use a comma and when to use a semicolon. Use a comma to separate an insubordinate clause from a subordinate clause. For example, "When I went to bed, the birds were just starting to chirp outside." Use a semicolon to separate two independent clauses. For example, "She wore her best dress that night; she wanted it to be special."