CLAUSE

In grammar, a clause is the smallest grammatical unit that can express a complete proposition[1]. In some languages it may be a pair or group of words that consists of a subject and a predicate, although in other languages in certain clauses the subject may not appear explicitly as a noun phrase, being instead marked on the verb (this is especially common in null subject languages). The most basic kind of sentence consists of a single (independent) clause. More complex sentences may contain multiple clauses, including clauses contained within clauses. Clauses may be independent or dependent. Independent clauses are those that could stand as a sentence by themselves, although they may be used connected with other clauses in a longer sentence. Dependent clauses are those that would be awkward or nonsensical if used alone, and must be used in a sentence also containing an independent clause.
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. However, modern linguists do not draw the same distinction, as they accept the idea of a non-finite clause, a clause that is organized around a non-finite verb.

[edit] Functions of dependent clauses
There are three main types of dependent clause: noun clauses, adjective clauses, and adverb clauses, so-called for their syntactic and semantic resemblance to nouns, adjectives, and adverbs, respectively. In the following English examples, dependent noun clauses are shown in bold:

• "I imagine (that) they are having a good time."
• "I keep thinking about what happened yesterday."

(The word that is optional in the first sentence, highlighting a complication in the entire dependent/independent contrast: "They are having a good time." is a complete sentence, and therefore an independent clause, but "(that) they are having a good time" is a dependent clause in the above example, with or without the conjunction "that".)
An adjective clause modifies a noun phrase. In English, adjective clauses typically come at the end of their phrase and usually have a relative pronoun forming a relative clause. The pronoun can sometimes be omitted to produce a reduced relative clause:

• "The woman (whom) I saw said otherwise."
• "I found the book that she suggested to me."

An adverb clause typically modifies its entire main clause. In English it usually precedes (in a periodic sentence) or follows (in a loose sentence) its main clause. The following adverb clauses show when (with the subordinating conjunction "when") and why (with the subordinating conjunction "because"):

• "When she gets here, all will be explained."
• "She's worried because they were already an hour late."

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 modifying "movie" (so "movie" is the antecedent of "which") ("We saw a movie. After the movie, we went dancing.") or as an adverb clause modifying the entire independent clause (so the independent clause in its entirety is the antecedent of "which") ("We saw a movie. After we saw the movie, we went dancing."). Sometimes the two interpretations are not synonymous, but are both 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).
[edit] Structures of dependent clauses
Dependent clauses may be classified by their structure, although this classification scheme does make some reference to the clause's function in a sentence. This scheme is more complex than analysis by function, as there are many different ways that a dependent clause can be structured. In English. Common structures include the following:

• Many dependent clauses, such as "before he comes" or "because they agreed," consist of a preposition-like subordinating conjunction, plus what would otherwise be an independent clause. These clauses act much like prepositional phrases, and are either adjective clauses or adverb clauses, with many being able to function in either capacity.
• Relative clauses, such as "which I couldn't see," generally consist of a relative pronoun, plus a clause in which the relative pronoun plays a part. Relative clauses usually function as adjective clauses, but occasionally they function as adverb clauses; in either case, they modify their relative pronoun's antecedent and follow the phrase or clause that they modify.
• Fused relative clauses, such as "what she did" (in the sense of "the thing she did"), are like ordinary relative clauses except that they act as noun clauses; they incorporate their subjects into their relative pronouns.
• Declarative content clauses, such as "that they came," usually consist of the conjunction that plus what would otherwise be an independent clause, or of an independent clause alone (with an implicit preceding that). For this reason, they are often called that-clauses. Declarative content clauses refer to states of affairs; it is often implied that the state of affairs is the case, as in "It is fortunate that they came," but this implication is easily removed by the context, as in "It is doubtful that they came."
• Interrogative content clauses, such as "whether they came" and "where he went" (as in "I don't know where he went"), are much like declarative ones, except that they are introduced by interrogative words. Rather than referring to a state of affairs, they refer to an unknown element of a state of affairs, such as one of the participants (as in "I wonder who came") or even the truth of the state (as in "I wonder whether he came").
• Small clauses, such as "him leave" (as in "I saw him leave") and "him to leave" (as in "I wanted him to leave"), are minimal predicate structures, consisting only of an object and an additional structure (usually an infinitive), with the latter being predicated to the former by a controlling verb or preposition.

TYPES OF CLAUSE
A clause is essentially a phrase, but with both a subject and predicate (more on those in the next post). Clauses are either dependent or independent. An independent clause can exist by itself as a complete sentence (as in “I love grammar.“), while a dependent clause cannot.
Dependent or Subordinate Clauses
A dependent or subordinate clause depends on an independent clause to express its full meaning (as in “Because I love grammar.”). These clauses begin with a dependent word, like a subordinating conjunction or a relative pronoun.
Dependent clauses can function as nouns, adjectives, and adverbs:

• Noun Clause – “The boy wondered if his parents bought him what he wanted for Christmas.” A noun clause can replace any noun in a sentence, functioning as a subject, object, or complement (see English Grammar: Basic Sentence Elements).

• Adjective Clause (or relative clause) – “I listened to the song that you told me about.” An adjective clause describes a noun just like an adjective. Which song? The new song, the good song, the song that you told me about. Often called relative clauses, they’re either restrictive or nonrestrictive (also called defining and non-defining, essential and nonessential, or integrated and supplementary):
  • Restrictive Clause – “The building that they built in San Francisco sold for a lot of money.” A restrictive clause begins with a relative pronoun like that or who (or sometimes which – see Which Versus That). It specifies or restricts the noun; in this case, it specifies which building the speaker is referring to. Note: the relative pronoun is often omitted (“The building (that) they built”), leaving what is called an elliptical clause or contact clause.
  • Nonrestrictive Clause – “The building, which they built in San Francisco, sold for a lot of money.” A nonrestrictive clause begins with a relative pronoun like which or who. It adds extra information about an already-specific noun; in this case, there’s only one building to talk about, whereas the example for the restrictive clause implies that there could be several buildings.

• Adverb Clause – “I’ll do the laundry when I’m out of clothes.” Like all adverbials, adverb clauses express when, where, why, and how something occurs. A dependent clause is an adverb clause if you can replace it with an adverb, as in “I’ll do the laundry later.”

Note: appositives can include clauses, but I’ve yet to find a source mentioning an “appositive clause.” They’re generally regarded as a type of noun phrase, even though they can be restrictive or nonrestrictive like relative clauses.