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In syntactic analysis, a 'constituent' is a word or a group of words that functions as a single unit within a hierarchical structure.
Phrases (noun phrases, verbal phrases, etc.) are usually constituents of a clause, but clauses may also be embedded into a bigger structure. For example, in the clause "I didn't hear what you said," the subordinate clause "what you said" is embedded into the main clause and is syntactically its object; this can be demonstrated by substituting the pronoun "it" for the subordinate clause "I didn't hear it."

Contents

Constituency tests

Substitution (replacement)

Movement

The stand-alone (or question) test

Other tests

Constituency tests and disambiguation

References

Constituency tests

Various constituency tests exist. Some syntacticians arrange such tests on a scale of reliability, with less-reliable tests treated as useful to confirm constituency though not sufficient on their own^[1].
Failing to pass a test, however, does not always mean that the unit is not a constituent. It is best to apply as many tests as possible to a given unit in order to prove or to rule out its constituency.

Substitution (replacement)

Using "it" instead of the whole clause "what you said" is called 'substitution', or 'replacement'. This is one of the tests used to determine the internal structure of a sentence, i.e. to determine its constituents. Substitution normally involves using pronouns like ''it'', ''he'', ''there'', ''here'' etc. in place of a phrase or a clause. If such a change yields a grammatical sentence where the general structure has not been altered, then the sequence of words which is being tested is a constituent:
e.g. ''I don't know the man who is sleeping in the car''.
:''I don't know him who is sleeping in the car''. (ungrammatical)
:''I don't know him.''
The ungrammaticality of the first changed version and the grammaticality of the second one demonstrates that the whole sequence, ''the man who is sleeping in the car'', and not just ''the man'' is a constituent functioning as a unit.

Movement

Movement includes such operations as clefting, fronting, pseudo-clefting and passivization.
'Fronting' is the simplest movement operation when the sequence we want to test is moved to the front of the sentence:
:''He is going to attend another language course to improve his English''.
:''To improve his English, he is going to attend another course''.
'Clefting' involves placing a sequence of words X within the structure beginning with "It is/was": ''It was X that...''
:''She bought a pair of gloves with silk embroidery''.
:''It was a pair of gloves with silk embroidery that she bought''.
'Pseudo-clefting' (also 'preposing') is similar to clefting in that it puts emphasis on a certain phrase in a sentence. It involves inserting a sequence of words before "is/are what" or "is/are who":
:''A pair of gloves with silk embroidery is what she bought''.
'Passivization' involves more than just movement. Apart from putting the object in the subject position and the subject after the preposition ''by'', it also triggers changes in the verb form:
:''A car driving at breakneck speed nearly hit the little dog''.
:''The little dog was nearly hit by a car driving at breakneck speed''.
In case passivization results in a grammatical sentence, the phrases that have been moved can be regarded as constituents.

The stand-alone (or question) test

This test refers to the ability of a sequence of words to stand alone as a reply to a question. It is often used to test the constituency of a verbal phrase but can also be applied to other phrases:
:''What did you do yesterday? - Worked on my new project.''
:::::::vs.
:''What did you do yesterday? - Worked on.'' (ungrammatical, which means that [worked on] is not a unit).
Linguists do not agree whether passing the stand-alone test is sufficient, though at a minimum they agree that it can help confirm the results of another constituency test¹.

Other tests

Other constituency tests can be used in a limited number of syntactic environments:

★ 'Deletion' checks whether a sequence of words can be omitted without influencing the grammaticality of the sentence — in most cases local or temporal adverbials can be safely omitted and thus constitute a syntactic unit;

★ 'Coordination' relies on the fact that only constituents can be coordinated, i.e., joined by means of the coordinating conjunction "and" (e.g., ''He enjoys [writing short stories] and [reading them to his friends].'')

Constituency tests and disambiguation

'Syntactic ambiguity' characterizes sentences which can be interpreted in different ways depending solely on how one perceives syntactic connections between words and arranges them into phrases.
:Possible interpretations of the sentence ''They killed the man with a gun'': a) The man was shot;
:b) the man who was killed had a gun with him.
The ambiguity of this sentence results from 2 possible arrangements into constituents:
:a) [They] {killed [the man] [with a gun]}.
:b) [They] {killed [the man with a gun]}.
In a) ''with a gun'' is an independent constituent with instrumental meaning , in b) it is embedded into the noun phrase ''the man with a gun'' modifying the noun ''man''. The autonomy of the unit ''with a gun'' in the first interpretation can be tested by the Stand-Alone test:
:''How did they kill the man? - With a gun.''
However, the same test can be used to prove that ''the man with a gun'' in b) should be treated as a unit:
:''Who(m) did they kill? - The man with a gun.''

References

General references:

★ Burton-Roberts, N. (1997) ''Analysing Sentences: An Introduction to English Syntax.'' 2nd Edition. Longman. pp. 7-23

★ Carnie, A. (2002) ''Syntax: A Generative Introduction.'' Oxford: Blackwell. pp. 51-53
Specific references:
1. April 22, 2006 Language Log posting by Eric Bakovic of University of California, San Diego