What Is a Subordinate Clause?
A subordinate clause, also called a dependent clause, consists of information to be combined with a main clause to form a single sentence. It resembles a main clause except for the presence of a subordinate conjunction or a relative pronoun, either of which renders it subordinate. Here are some guidelines about its use.
To convey two thoughts, you can write, for example, “I went to school. I was feeling ill.” However, this construction offers no clue to the relationship between the two statements. To clarify that you wish to explain that you attended school in spite of your illness, you can combine the two thoughts into one, but you must include a conjunction that conveys the subordinate status of the second statement: “I went to school, although I was feeling ill.” (The subordinate clause can also come first: “Although I was feeling ill, I went to school.”) Other common subordinate conjunctions include because, until, and when.
Note that no other internal punctuation can substitute for the comma: The subordinate conjunction is not necessary in the following sentence, because the semicolon indicates that the sentence consists of two main clauses: “I went to school; I was feeling ill.” (You could also separate the statements with an em dash or enclose the second sentence in parentheses.) However, without the subordinate conjunction, the relationship between the two statements is not expressed. The sample sentence suggests that you went to school because you were feeling ill, not in spite of your illness.
To combine the thoughts “I went to school even though I was ill” and “Going to school when I was ill was not a good idea,” you can substitute the subject of the second statement with a relative pronoun: “I went to school even though I was ill, which was not a good idea.” (Other relative pronouns include that, who, and whose.)
Because this type of subordinate clause incorporates a relative pronoun, it is also called a relative clause. Note that you must take care when constructing some relative clauses, because the meaning can differ depending on whether the relative pronoun is preceded by a comma.
For example, “We got a ride with our friends who were also going to the movie” implies that among all our friends, we rode with those who were also going to the movie. However, in “We got a ride with our friends, who were also going to the movie,” the relative clause refers to a particular group of friends and makes no distinction between them and other friends because it makes no reference to other friends at all.
In the first sentence, the relative clause is a required part of the sentence and thus is called an essential relative clause. In the second sentence, it is not required (you can omit the relative clause and simply write, “We got a ride with our friends”) and thus is called a nonessential relative clause.
When a clause follows and expands on a verb (or, sometimes, an adjective or another adverb), it can also be called an adverbial clause. For example, in “They laughed because the dog was chasing its tail,” the phrase after laughed explains the reason for the laughter. However, a sentence such as “They laughed at the dog” contains not an adverbial clause but an adverbial phrase, because “at the dog” is not a clause; unlike “the dog was chasing its tail,” it does not constitute a complete thought.
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