Some sentences are made of single clauses. For example, This clause is a sentence. Others are made of multiple clauses. For instance, this sentence has three clauses:
Since no one could pick us up, we had to take a cab, which cost almost $70.
A clause is different from a phrase in that it is a self-contained unit with both a subject and a predicate, while a phrase is simply any group of words that work together somehow.
An independent clause is a clause that is syntactically self-contained. It could stand as a complete sentence on its own.
She’s a dog, and I’m a chicken.
A dependent clause (also known as a subordinate clause) is a clause that could not stand alone as a sentence. Dependent clauses depend on main clauses to supply context, meaning, and completion
Dependent clauses and commas
When a dependent clause precedes a main clause, use a comma to separate the the clauses—for example:
When he gets a shot, he always cries.
When a dependent clause follows the main clause, use no comma:
He always cries when he gets a shot.
Adverbial clauses are clauses that modify verbs, adjectives, and adverbs. Or, put more simply, an adverbial clause functions as an adverb. For example, in the sentence, “He stole my book while I wasn’t looking,” while I wasn’t looking is an adverbial clause because it is a clause that modifies the verb stole.
Adverbial clauses usually modify verbs. They can do many things in this capacity—for instance, an adverbial clause can express where or when an action denoted by a verb is performed:
Everywhere I look, I see your face. [Everywhere I look modifies see.]
The cat was sleeping when I got home. [When I got home modifies was.]
Adverbial clauses may provide causes or purposes:
He cried because you were mean to him. [Because you were mean to him modifies cried.]
She opened the curtains so he would have a little more light. [So he would have a little more light modifies opened.]
They may make concessions:
We went to the movie even though we knew it would be bad. [Even though we knew it would be mad modifies went.]
And they may express conditions:
I’ll get the tab if you pay the tip. [If you pay the tip modifies get.]
Dr. Horton said the advocates, whom he declined to name, wanted the new information held and released. [New York Times]
In this sentence, whom he declined to name is a relative clause modifying the noun advocates.
Relative clauses are usually introduced by relative pronouns. In English, the relative pronouns are that, which, who, and whom. For example, a relative pronoun (boldfaced) introduces the relative clause (underlined) in each of these sentences:
(1) This is the book that I read on the plane.
(2) I bought a book, which I then read on the plane.
(3) I said hello to the flight attendant who took my ticket.
(4) The man sitting beside me, whom I’d seen in the airport, was using his laptop.
The relative pronoun that can sometimes be omitted—for example:
This is the book I read on the plane.
Restrictive vs. nonrestrictive dependent clauses
Relative clauses that are essential to the meaning of the sentence are restrictive. Those that could be omitted without major loss of meaning are nonrestrictive.
A nonrestrictive adjective clause should be separated from the main clause by a comma (if the clause begins or ends a sentence) or commas (if the clause is in the middle of a sentence). When the clause modifies a nonperson, which is usually used. Here are a few examples, with the nonrestrictive relative clause underlined:
The item, which was slightly larger than a hockey puck, is called a Haitian cookie. [Vindy]
The flavourless lobster meat doesn’t hold up to the rest of the dish, which is crunchy, lightly tangy and satisfying. [FFWD Weekly (now offline)]
The woman, who said she no longer had the money, took her two young children and spent the night at her mother’s house. [Cleveland.com]
A restrictive adjective clause should not be set apart with commas. Clauses modifying nonpersons usually use that—though, again, that may sometimes be omitted. In these examples, the restrictive adjective clause is underlined:
The earthquake that recently hit Haiti is the worst in more than 200 years. [The Daily Tell]
The lesson I learned involved two different auto shops. [Acroschool.com]
About half the people who ski in the area wear helmets … [LA Times]