In linguistics and grammar, a sentence is a linguistic expression, such as the English example "The quick brown fox jumps over the lazy dog." In traditional grammar, it is typically defined as a string of words that expresses a complete 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 graphological features 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.
A sentence is typically associated with a clause and a clause can be either a clause simplex or a clause complex. A clause is a clause simplex if it represents a single process going on through time and it is a clause complex if it represents a logical relation between two or more processes and is thus composed of two or more clause simplexes.
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 phrases (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. Notice that 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.
Sentences can also be classified based on the speech act which they perform. For instance, English sentence types can be carved up as follows:
The form (declarative, interrogative, imperative, or exclamative) and meaning (statement, question, command, or exclamation) of a sentence usually match, but not always.[unreliable source?] 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 express a statement on the listener's lack of ability; see rhetorical question.
A major sentence is a regular sentence; it has a subject and a predicate, e.g. "I have a ball.". In this sentence, one can change the persons, e.g. "We have a ball.". However, a minor sentence is an irregular type of sentence that does not contain a main clause, e.g. "Mary!", "Precisely so.", "Next Tuesday evening after it gets dark.". Other examples of minor sentences are headings (e.g. the heading of this entry), stereotyped expressions ("Hello!"), emotional expressions ("Wow!"), proverbs, etc. These can also include nominal sentences like "The more, the merrier". These mostly omit a main verb for the sake of conciseness, but may also do so in order to intensify the meaning around the nouns.
The 1980s saw a renewed surge in interest in sentence length, primarily in relation to "other syntactic phenomena".
One definition of the average sentence length of a prose passage is the ratio of the number of words to the number of sentences.[unreliable source?] 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.
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".
Incidentally, you might be interested to know that sometimes we have a mismatch between the structure of a sentence, and how it is used. If I say to you at dinner Can you pass me the salt? then this is technically a question that has ‘yes’ or ‘no’ as possible answers, but in fact the pattern is used here to tell you to do something. It’s because of these possible mismatches that linguists prefer to use two sets of terms to describe the phenomena above. For the sentence patterns they use declarative, interrogative, imperative and exclamative. Corresponding to this are statement, question, command and exclamation to describe how these patterns are used.
(Caption) Table 26.3: Sentence length (expressed by the number of clauses) and clause length (expressed by the number of phones) in a Turkish text