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A grammatical constituent is a unit that participates as a component of a larger grammatical construction. Constituency describes hierarchical grouping, so adjacency alone does not establish that an arbitrary sequence of words forms one unit. Different levels of an analysis can contain constituents of different sizes.^[ref]

In The red kite rose, a conventional analysis groups the red kite as a noun phrase and assigns it the subject relation. Replacing the whole group with it gives It rose, providing one structural diagnostic. The example separates the number of written words from the number of units occupying that grammatical position.

A useful analysis combines several diagnostics and states the construction under consideration. Context can license omissions or exceptional patterns, so one substitution does not prove every proposed grouping. Grammatical constituents provide the units of a theory of syntax; judgments and controlled comparisons supply evidence for choosing among theories.

A noun phrase is a grammatical unit conventionally analyzed as headed by a noun, potentially together with dependents that modify or specify it. A single noun or pronoun can also form a minimal noun phrase under this analysis.^[ref]

In The small bird landed, the words the small bird form a noun phrase containing the head bird, the adjective small and a determiner. The whole unit occupies the subject position. Replacing it with it gives It landed, illustrating that a multiword expression can occupy a position also available to one word.

Such substitution is useful evidence for a grammatical constituent, but one test alone does not settle every structural question. A clear syntax states how the proposed head and dependents combine and tests the same account across further constructions. Counting spaces therefore measures a different property from counting noun phrases.

A subordinate clause is a clause embedded within a larger grammatical construction, where it supplies part of the structure or interpretation. It contrasts with a main clause in that construction. Subordination describes a relationship among units, rather than merely identifying whichever words occur after a comma.^[ref]

In the original example The sensor stopped because the battery failed, because the battery failed supplies a reason for the main assertion. In I know that the sensor stopped, the embedded material instead supplies what is known. Both contain their own subject-predicate organization, while contributing differently to the larger expression.

A structural analysis can vary the embedded content while preserving its role in the surrounding syntax. Comparing that pattern across examples helps distinguish an embedded grammatical constituent from an unrelated second assertion. The analysis must also keep the grammatical embedding separate from whether the speaker actually believes the embedded content.