22.01.03 · grammar / sentence-structure

Sentences: subject and predicate

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Anchor (Master): Chomsky 1957, Syntactic Structures; Pollard & Sag 1994, Head-Driven Phrase Structure Grammar

Intuition [Beginner]

A sentence is a complete thought. It has two parts: a subject (who or what the sentence is about) and a predicate (what the subject does or is).

"The bird sings." Subject = the bird. Predicate = sings.

"The old man walked slowly to the store." Subject = the old man. Predicate = walked slowly to the store. The predicate can be long -- it includes the verb and everything that goes with it.

Every sentence needs both. "The bird" by itself is not a sentence -- nothing is happening. "Sings" by itself is not a sentence either -- who sings? Put them together and you have a complete thought.

Visual [Beginner]

Sentence
+-------------------------------+
| Subject        | Predicate    |
| (who/what)     | (action/state)|
+-------------------------------+
| The dog        | barks.       |
| My sister      | reads books. |
| Rain           | falls.       |
| The sky        | is clear.    |
+-------------------------------+

The dividing line falls between the subject and the verb. Everything before the verb is the subject (or part of it). The verb and everything after it form the predicate.

Worked example [Beginner]

Split this sentence into subject and predicate: "The small puppy chewed the new shoe."

Subject: "The small puppy" -- who the sentence is about. Predicate: "chewed the new shoe" -- what the subject did.

To find the subject, ask "who?" or "what?" before the verb. What chewed the shoe? The small puppy. To find the predicate, find the verb and take everything from there to the end.

What this tells us: every complete sentence can be split this way. If it cannot, it is either a fragment (missing a piece) or a run-on (two sentences jammed together without a proper join).

Check your understanding [Beginner]

Formal definition [Intermediate+]

A sentence is a grammatically complete construction consisting of at least one subject and one predicate. The subject is a noun phrase (or pronoun) that identifies what the sentence is about. The predicate is a verb phrase that says something about the subject.

Sentence types by purpose

Type	Function	End mark	Example
Declarative	States a fact or opinion	Period	The sky is blue.
Interrogative	Asks a question	Question mark	Is the sky blue?
Imperative	Gives a command	Period or exclamation	Look at the sky.
Exclamatory	Expresses strong emotion	Exclamation mark	How blue the sky is!

Sentence types by structure

Type	Structure	Example
Simple	One independent clause	The dog barked.
Compound	Two independent clauses joined	The dog barked, and the cat ran.
Complex	One independent + one or more dependent clauses	The dog barked because it heard a noise.
Compound-complex	Two+ independent + one+ dependent	The dog barked, and the cat ran because it was scared.

The subject

The subject is typically a noun phrase. It can be:

A single noun or pronoun: "Birds fly."
A noun with modifiers: "The large brown eagle soared."
A gerund phrase: "Swimming in the ocean is dangerous."
A noun clause: "What she said surprised everyone."
An infinitive phrase: "To leave now would be rude."

In English, the subject usually comes before the verb (SVO word order), but inversions exist: "Down came the rain" (verb before subject). Questions invert subject and auxiliary: "Has she arrived?" (auxiliary before subject).

The predicate

The predicate is centred on the verb and includes everything else in the clause that is not the subject:

Verb only: "It rained." (predicate = rained)
Verb + direct object: "She read the book." (predicate = read the book)
Verb + indirect object + direct object: "He gave her flowers." (predicate = gave her flowers)
Verb + subject complement: "The cake smells delicious." (predicate = smells delicious)

Fragments and run-ons

A fragment is an incomplete sentence -- it is missing a subject, a predicate, or both. "Because it rained." (fragment -- what happened because it rained?) Fragments are common in speech and creative writing but are errors in formal writing.

A run-on joins two independent clauses without proper punctuation or conjunction. "It rained the game was cancelled." Two sentences fused. Fixed with a period ("It rained. The game was cancelled."), a semicolon ("It rained; the game was cancelled."), or a conjunction ("It rained, so the game was cancelled.").

Counter-examples to common slips

Commands have an implied subject. "Close the door" is a complete sentence. The subject is "you" (understood). The predicate is "close the door."
"There" and "it" can be dummy subjects. "There is a problem" -- "there" is a dummy subject; the real subject is "a problem." "It is raining" -- "it" is a dummy subject with no referent.
Not every period marks the end of a sentence. Abbreviations (Dr., St., etc.) contain periods but do not end sentences.

Linguistic theory [Master]

Constituency and phrase structure

The subject-predicate split corresponds to a deeper structural claim: sentences have constituent structure. The sentence $S$ decomposes into a noun phrase $N P$ (the subject) and a verb phrase $V P$ (the predicate):

$S \to N P V P$

This is the fundamental rewrite rule of phrase structure grammar (Chomsky 1957). The $V P$ itself decomposes: $V P \to V N P$ (for transitive verbs), $V P \to V P P$ (for verbs with prepositional complements), and so on. The resulting tree structure captures the intuition that "the tall woman in the red coat" forms a single unit (constituent) that behaves differently from the separate words taken individually.

Evidence for constituency. Constituency is not just a theoretical claim. It is supported by distributional tests:

Substitution: a constituent can be replaced by a single word. "The tall woman in the red coat opened the door" $\to$ "She opened the door." The pronoun "she" replaces the entire subject NP.
Movement: a constituent can be moved as a unit. "She opened the door" $\to$ "The door, she opened." The object NP moves to the front as a unit.
Coordination: constituents of the same type can be joined. "She opened the door and closed the window." Two VPs coordinated.

X-bar theory

The simple $S \to N P V P$ rule hides internal structure. X-bar theory (Jackendoff 1977, Chomsky 1981) proposes that every phrase has a three-level structure:

$X P \to Specifier, X^{'}$ $X^{'} \to X, Complement$

Where $X$ is the head of the phrase (a noun for NP, a verb for VP), $X^{'}$ is the intermediate projection, and $X P$ is the maximal projection. For the noun phrase "the tall woman":

Head ( $N$ ): woman
Complement: (none in this case)
Adjunct: tall (modifies the head)
Specifier: the (determiner)

This theory predicts cross-categorial uniformity: noun phrases, verb phrases, adjective phrases, and prepositional phrases all share the same internal geometry. The prediction is borne out across languages.

Dependency grammar

An alternative to constituency is dependency grammar (Tesniere 1959), which dispenses with phrase nodes entirely. Instead, each word is a node, and edges connect heads to dependents:

opened (root)
  |-- subject: woman
  |     |-- det: the
  |     |-- adj: tall
  |-- object: door
        |-- det: the

"Opened" is the root. "Woman" depends on "opened" (as its subject). "The" depends on "woman" (as its determiner). No phrase nodes are needed. Dependency grammar underlies most modern NLP parsing (Universal Dependencies, de Marneffe et al. 2021) and connects to the graph-theoretic formalisms used elsewhere in the curriculum.

The verb-second constraint (comparative)

English is largely SVO, but German, Dutch, and the Scandinavian languages enforce verb-second (V2) word order: in main clauses, the finite verb must occupy the second position, regardless of what comes first.

German: "Heute gehe ich ins Kino" (Today go I to the cinema)
English: "Today I am going to the cinema"

The verb moves to a structural position (C, the complementiser head in generative syntax) that English fills only in questions and certain inversions. This parametric difference is a core example in comparative syntax.

Connections

The $S \to N P V P$ decomposition parallels the function application in the lambda calculus: if the verb is a predicate $λ x . P (x)$ and the subject NP provides an argument $a$ , then the sentence asserts $P (a)$ . This is Montague's insight (1970s): natural language composition is function application. The connection to 00.02.05 (functions) and the formal logic strand is direct.

Historical context [Master]

The subject-predicate analysis descends from Aristotle's Peri Hermeneias (On Interpretation), where every statement is analysed as a combination of a subject (hypokeimenon, "that which underlies") and a predicate (katégoroumenon, "that which is said of it"). The binary split survived through the Port-Royal Grammar (1660), traditional school grammar, and into modern linguistics, though X-bar theory and dependency grammar have refined it considerably. The insight remains: a sentence asserts that a predicate holds of a subject. Everything else is detail about how that assertion is structured.

Bibliography [Master]

Chomsky, N. (1957). Syntactic Structures. Mouton.
Jackendoff, R. (1977). X-bar Syntax: A Study of Phrase Structure. MIT Press.
Pollard, C. & Sag, I. A. (1994). Head-Driven Phrase Structure Grammar. University of Chicago Press.
Tesniere, L. (1959). Elements de syntaxe structurale. Klincksieck.
de Marneffe, M.-C. et al. (2021). "Universal Dependencies." Computational Linguistics, 47(2).