According to certain traditional linguistic theories, a subordinate clause or dependent clause is a clause that provides a sentence element with additional information, but which cannot stand as a sentence. A dependent clause can either modify an adjacent clause or serve as a component of an independent clause.
In Indo-European languages, a dependent clause usually begins with a dependent word. One kind of dependent word is a subordinating conjunction. Subordinating conjunctions are used to begin dependent clauses known as adverbial clauses, which serve as adverbs. In the following examples, the adverbial clauses are bold and the subordinating conjunctions are italicized:
A subordinating conjunction can also introduce a noun clause:
Another type of dependent word is the relative pronoun. Relative pronouns begin dependent clauses known as relative clauses; these are adjective clauses, because they modify nouns. In the following example, the relative clause is bold and the relative pronoun is italicized:
A relative adverb plays the part of an adverb in a relative clause, as in
An interrogative word can serve as an adverb in a noun clause, as in
A content clause, also known as a "noun clause", provides content implied or commented upon by its main clause. It can be a subject, predicate nominative, direct object, appositive, indirect object, or object of the preposition. Some of the English words that introduce content clauses are that, who (and formal whom), whoever (and formal whomever), whether, why, what, how, when, and where. Notice that some of these words also introduce relative and adverbial clauses. A clause is a content clause if a pronoun (he, she, it, or they) could be substituted for it.
In English, in some instances the subordinator that can be omitted.
In Indo-European languages, a relative clause, also called an adjectival clause or an adjective clause, meets three requirements:
The adjective clause in English will follow one of these patterns:
For a discussion of adjective clauses in languages other than English, see Relative clause#Examples.
The punctuation of an adjective clause depends on whether it is essential (restrictive) or nonessential (non-restrictive) and uses commas accordingly. Essential clauses are not set off with commas; nonessential clauses are. An adjective clause is essential if the information it contains is necessary to the meaning of the sentence:
The word "vegetables" is non-specific. Accordingly, for the reader to know which are being mentioned, one must have the information provided in the adjective clause (in italics). Because it restricts the meaning of "vegetable", the adjective clause is called a restrictive clause. It is essential to the meaning of the main clause and uses no commas (and so does not experience a pause when spoken).
However, if the additional information does not help to identify more narrowly the identity of the noun antecedent but rather simply provides further information about it, the adjective clause is nonrestrictive and so requires commas (or a spoken pause) to separate it from the rest of the sentence:
Depending on context, a particular noun could be modified by either a restrictive or nonrestrictive adjective clause. For example, while "broccoli" is modified nonrestrictively in the preceding sentence, it is modified restrictively in the following.
"He saw Mary when he was in New York" and "They studied hard because they had a test" both contain adverbial clauses (in italics). Adverbial clauses express when, why, where, opposition, and conditions, and, as with all dependent clauses, they cannot stand alone. For example, When he was in New York is not a complete sentence; it needs to be completed by an independent clause, as in:
A complex sentence contains an independent clause and at least one dependent clause. A sentence with two or more independent clauses plus (one or more) dependent clauses is referred to as a compound-complex sentence. (Every clause contains a subject and predicate.) Here are some English examples:
My sister cried because she scraped her knee. (complex sentence)
When they told me (that) I won the contest, I cried, but I didn't faint. (compound-complex sentence)
This sentence contains two dependent clauses: "When they told me", and "(that) I won the contest", the latter which serves as the object of the verb "told". The connecting word "that", if not explicitly included, is understood to implicitly precede "I won" and in either case functions as a subordinating conjunction. This sentence also includes two independent clauses, "I cried" and "I didn't faint", connected by the coordinating conjunction "but". The first dependent clause, together with its object (the second dependent clause), adverbially modifies the verbs of both main clauses.
In these cases, the subject of the dependent clause may take a non-nominative form. An example is: