LT5903 Chap.5 Constituency, Rules and Syntactic Ambiguity
Constituency, Rules and Syntactic Ambiguity
Evidence for structure
Syntax asks how words and phrases combine. Co-occurrence distinguishes required arguments from optional adjuncts; word order and agreement constrain combinations. Constituency tests identify expressions that behave as units, but no single test works in every case.
Phrase-structure rules capture categories and order, support generation and parsing, and distinguish head-initial from head-final patterns.
Structural ambiguity arises when the same words fit more than one tree, including attachment, lexical-category, premodifier and coordination ambiguity.
Distinguish category from grammatical role
A noun phrase is a category of constituent, while subject or object describes its role in a larger structure. The same category can occupy different roles, and a role may be realised by more than one form.
Begin by identifying heads and the expressions that depend on them. Distribution offers evidence for category: items that occur in comparable frames may belong together. Agreement and word order provide further constraints.
Avoid assigning a category from meaning alone, because words with similar meanings can behave differently in sentences.
Use several constituency diagnostics
Substitution asks whether a string can be replaced by an appropriate pro-form. Movement asks whether it can relocate as a unit, coordination asks whether it can join a comparable expression and answer fragments ask whether it can stand as a response.
Each test has limitations, so convergence is stronger than one result. A failed movement may reflect an independent restriction rather than non-constituency. State the transformed sentence, judge its acceptability and explain what grouping the result supports.
Do not merely name the test.
Separate arguments from adjuncts
Arguments are selected by a head and help complete its valency, while adjuncts add optional modification and are less tightly selected. Optionality alone is imperfect because understood arguments can be omitted in some contexts. Consult meaning, co-occurrence restrictions, repeatability and the head's lexical requirements together.
In a tree, an argument typically combines closer to the head than an adjunct. A useful answer describes what participant or complement the head requires and what extra information an optional modifier contributes.
Build and audit phrase-structure trees
Every node should have a category label and every terminal should appear once in the original order.
Begin with the smallest clear constituents, combine them under rules and ensure the result reaches the start category. Head direction is a property of the language or construction, not a universal assumption. After drawing, read each branch as a rule and verify that the grammar licenses it.
A tree is an explicit hypothesis: missing nodes, crossed branches or unlabeled groups make its predictions impossible to inspect.
Show ambiguity through two structures
Lexical ambiguity assigns different senses or categories to a word, whereas structural ambiguity keeps the word sequence but changes grouping.
For prepositional-phrase attachment, place the phrase inside the noun phrase for one reading and under the verb phrase for another. Coordination and premodifier ambiguity require comparable rebracketing. Give a paraphrase for each tree so the structural difference has a visible interpretive consequence. If the two drawings make the same grouping, they do not explain two readings even if their prose labels differ.
What this chapter covers
- 01
Arguments and adjuncts
- 02
Word order and agreement
- 03
Constituency tests
- 04
Phrase-structure rules
- 05
Structural ambiguity
Worked example · free
Draw two attachment sites
- 2State both paraphrases.
- 3Attach the phrase inside the noun phrase.
- 3Attach it to the verb phrase.
Key terms
- Argument
- A required co-occurring expression.
- Adjunct
- An optional modifier that can recur.
- Constituent
- An expression functioning as a syntactic unit.
- Head
- The core category determining a phrase's type.
- Attachment Ambiguity
- Ambiguity caused by more than one structural attachment site.
Constituency, Rules and Syntactic Ambiguity FAQ
Why should I use several constituency tests?
Question answers, clefting and pro-form substitution each have limitations. Converging results provide stronger evidence, while a failed cleft alone may reflect restrictions on that construction rather than absence of a constituent.
How do arguments differ from adjuncts?
Arguments are selected by a head and may be required to complete its frame. Adjuncts add optional information and can often recur. Test omission carefully because discourse can sometimes hide a semantically understood argument.
What makes an ambiguity syntactic?
A syntactic ambiguity keeps the same word meanings but permits more than one hierarchical structure. Draw the competing trees and identify the attachment or grouping difference; a paraphrase displays the result but does not itself explain it.
Exam move
For each sentence, identify the head and required dependants before testing optional strings. Practise drawing paired trees with identical words and one changed attachment site, then state the reading licensed by each. Maintain a constituency ledger that records the transformed sentence, acceptability judgement and limitation of every test.
Audit finished trees from terminals upward so each word appears once, every node is labelled and every branch corresponds to a stated rule.
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