In grammar, a clause is a word or group of words that consists of a subject and a predicate, although in some languages and some types of clauses, the subject may not appear explicitly as a noun phrase. It may instead be marked on the verb (this is especially common in null subject languages.) The most basic kind of sentence consists of a single clause; more complicated sentences may contain multiple clauses. Indeed, it is possible for one clause to contain another.
Clauses are often contrasted with phrases. Traditionally, a clause was said to have both a finite verb and its subject, whereas a phrase either contained a finite verb but not its subject (in which case it is a verb phrase) or did not contain a finite verb. Hence, in the sentence "I didn't know that the dog ran through the yard", "that the dog ran through the yard" is a clause, as is the sentence as a whole, while "the yard", "through the yard", "ran through the yard", and "the dog" are all phrases. Modern linguists do not draw quite the same distinction, however, the main difference being that modern linguists accept the idea of a non-finite clause, a clause that is organized around a non-finite verb.
Examples in English include the following:
One major way to classify dependent clauses is by function; that is, by the roles they play in the clauses they are subordinate to. Since the same dependent clause might have different roles in different sentences, this classification must be applied on a per-sentence basis.
Under this classification scheme, there are three main types of dependent clauses: noun clauses, adjective clauses, and adverb clauses, so called for their syntactic and semantic resemblance to noun phrases, adjective phrases, and adverbials, respectively. The exact uses of each vary somewhat from language to language, but a noun clause typically acts as the subject of a verb or as the object of a verb or preposition, as in these English examples:
(Note that the word that is optional in the second sentence, highlighting a complication in the entire dependent/independent contrast: "They're having a good time" is a complete sentence, and therefore an independent clause, but in "I imagine they're having a good time", it acts as a dependent clause.)
An adjective clause modifies a noun phrase. In English, adjective clauses typically come at the end of their noun phrases:
An adverb clause typically modifies its entire main clause. In English, it usually precedes or follows its main clause:
The line between categories may be indistinct, and, in some languages, it may be difficult to apply these classifications at all. At times more than one interpretation is possible, as in the English sentence "We saw a movie, after which we went dancing", where "after which we went dancing" can be seen either as an adjective clause ("We saw a movie. After the movie, we went dancing.") or as an adverb clause ("We saw a movie. After we saw the movie, we went dancing."). More complicated, sometimes the two interpretations are not synonymous, but both are intended, as in "Let me know when you're ready", where "when you're ready" functions both as a noun clause (the object of know, identifying what knowledge is to be conveyed) and as an adverb clause (specifying when the knowledge is to be conveyed).
The other major way to classify dependent clauses is by their structure, though even this classification scheme does make some reference to the clause's function in a sentence. This scheme is more complex, as there are many different ways that a dependent clause can be structured. In English, common structures include: