In linguistics and grammar, a sentence is a linguistic expression, such as the English example "The quick brown fox jumps over the lazy dog" (a pangram). In traditional grammar, it is typically defined as a string of words that expresses a thought, or as a unit consisting of a subject and predicate. In non-functional linguistics it is typically defined as a maximal unit of syntactic structure such as a constituent. In functional linguistics, it is defined as a unit of written texts delimited by writing such as upper-case letters and markers such as periods, question marks, and exclamation marks. This notion contrasts with a curve, which is delimited by phonologic features such as pitch and loudness and markers such as pauses; and with a clause, which is a sequence of words that represents some process going on throughout time.Halliday, M. A. K. and Matthiessen, C. M. I. M. 2004. An Introduction to Functional Grammar. Arnold: p. 6. A sentence can include words grouped meaningfully to express a statement, question, exclamation, request, Imperative mood, or suggestion.
A clause (simplex) typically contains a predication structure with a subject noun phrase and a finite verb. Although the subject is usually a noun phrase, other kinds of (such as gerund phrases) work as well, and some languages allow subjects to be omitted. In the examples below, the subject of the outmost clause simplex is in italics and the subject of boiling is in square brackets. There is clause embedding in the second and third examples.
There are two types of clauses: independent and non-independent/ interdependent. An independent clause realises a speech act such as a statement, a question, a command or an offer. A non-independent clause does not realise any act. A non-independent clause (simplex or complex) is usually logically related to other non-independent clauses. Together, they usually constitute a single independent clause (complex). For that reason, non-independent clauses are also called interdependent. For instance, the non-independent clause because I have no friends is related to the non-independent clause I don't go out in I don't go out, because I have no friends. The whole clause complex is independent because it realises a statement. What is stated is the causal nexus between having no friend and not going out. When such a statement is acted out, the fact that the speaker doesn't go out is already established, therefore it cannot be stated. What is still open and under negotiation is the reason for that fact. The causal nexus is represented by the independent clause complex and not by the two interdependent clause simplexes.
See also copula for the consequences of the verb to be on the theory of sentence structure.
The form (declarative, interrogative, imperative, or exclamative) and meaning (statement, question, command, or exclamation) of a sentence usually match, but not always. For instance, the interrogative sentence "Can you pass me the salt?" is not intended to express a question but rather to express a command. Likewise, the interrogative sentence "Can't you do anything right?" is not intended to express a question on the listener's ability, but rather to make an exclamation about the listener's lack of ability, also called a rhetorical question.
Sentences that comprise a single word are called word sentences, and the words themselves .
One definition of the average sentence length of a prose passage is the ratio of the number of words to the number of sentences. The textbook Mathematical Linguistics, by András Kornai, suggests that in "journalistic prose the median sentence length is above 15 words". The average length of a sentence generally serves as a measure of sentence difficulty or complexity. In general, as the average sentence length increases, the complexity of the sentences also increases.
Another definition of "sentence length" is the number of clauses in the sentence, whereas the "clause length" is the number of phones in the clause.
Research by Erik Schils and Pieter de Haan by sampling five texts showed that two adjacent sentences are more likely to have similar lengths than two non-adjacent sentences, and almost certainly have a similar length when in a work of fiction. This countered the theory that "authors may aim at an alternation of long and short sentences". Sentence length, as well as word difficulty, are both factors in the readability of a sentence; however, other factors, such as the presence of conjunctions, have been said to "facilitate comprehension considerably".
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