A clause is a group of words containing a subject and a predicate. There are two kinds of clauses: independent and dependent. Independent clauses (also known as main clauses) can stand alone as complete sentences.

The window is open.

Pairs of independent clauses may be joined with a coordinating conjunction and a comma.

The window is open, so the room feels cool.

Like independent clauses, dependent clauses (also referred to as subordinate clauses) contain a subject and a predicate. They cannot stand alone as complete sentences, however, for they begin with a subordinating word—a subordinating conjunction or a relative pronoun —that connects them to an independent clause.

Because the window is open, the room feels cool.

The subordinating conjunction because transforms the independent clause the window is open into a dependent clause. In doing so, it indicates a causal relationship between the two clauses.

Dependent clauses function as nouns, adjectives, or adverbs.

Identifying noun clauses

Noun clauses are always contained within another clause. They usually begin with a relative pronoun (that, which, what, who, whom, whose, whatever, whoever, whomever, whichever) or with when, where, whether, why, or how.

Like a noun, a noun clause is an integral part of the sentence; for example, in the second sentence the independent clause is not just She explained but She explained that the research was necessary. This complex sentence is built out of two sentences; one of them (The research was necessary) is embedded in the other (She explained [something]). The relative pronoun that introduces the noun clause that is the object of explained.

A that clause can serve as the subject of a sentence, but the effect is very formal:

In less formal contexts, and in spoken English, a long noun clause is usually moved to the end of the sentence and replaced with the “dummy subject” it.

It was surprising that the city had been abandoned.

Identifying adjective clauses

Adjective clauses modify nouns and pronouns in another clause. Usually, they immediately follow the words they modify.

Sometimes the relative pronoun introducing an adjective clause may be omitted, as in the following examples:

To see how the adjective clause fits into this sentence, rewrite it as two sentences: The company grew rapidly. The family had invested in it. To make The family had invested in it a relative clause, change it to a relative pronoun and move it to the beginning of the clause: The family had invested in it becomes that the family had invested in. Then position the new clause after the word it describes (in this case, company): The company that the family had invested in grew rapidly.

In very formal writing, when the pronoun you are changing is the object of a preposition, select which (or whom for people) and move the whole prepositional phrase to the beginning of the clause: The company in which the family had invested grew rapidly. In many American English contexts, however, such constructions may sound too formal, so consider your audience carefully.

Identifying adverb clauses

Adverb clauses modify verbs, adjectives, or other adverbs. They begin with a subordinating conjunction (36g). Like adverbs, they usually tell when, where, why, how, or to what extent.