Christel Kemke 2007/08 COMP 4060 Natural Language Processing Word Classes and English Grammar

Christel Kemke

COMP 4060 Natural Language Processing

Word Classes and

English Grammar

2007/08 Christel Kemke

English Grammar

Describing Natural Language Syntax: Word Classes and English Grammar

• Word Classes and Part-of-Speech Tagging

• CFG with Grammatical Extensions• Sentence Structures• Noun Phrase - Modifications• Verb Phrase - Subcategorization

Christel Kemke

Word Classes and POS Tagging


Word Classes

Sort words into categories according to: morphological properties

Which types of morphological forms do they take?e.g. form plural: noun+s; 3rd person: verb+s

distributional propertiesWhat other words or phrases can occur nearby?e.g. possesive pronoun before noun

semantic coherenceClassify according to similar semantic type. e.g. nouns refer to object-like entities


Open vs. Closed Word Classes

Open Class TypesThe set of words in these classes can

change over time, with the development of the language, e.g. spaghetti, to download

Open Class Types: nouns, verbs, adjectives, adverbs


Open vs. Closed Word Classes

Closed Class TypesThe set of words in these classes are very

much determined and hardly ever chnage for one language.

Closed Class Types: prepositions, determiners, pronouns, conjunctions, auxiliary verbs, particles, numerals


Open Class Words: Nouns

Nouns – denote objects, concepts, …

Proper NounsNames for specific individual objects, entitiese.g. the Eiffel Tower, Dr. KemkeCommon NounsNames for categories or classes or abstractse.g. fruit, banana, table, freedom, sleep, ...Count Nounsenumerable entities, e.g. two bananasMass Nounsnot countable items, e.g. water, salt, freedom


Open Class Words: Verbs

Verbs – denote actions, processes, states e.g. smoke, dream, rest, run several morphological forms e.g.

non-3rd person - eat3rd person - eatsprogressive/ - eating present participle/ gerundivepast participle - eaten

Auxiliaries, e.g. be, as sub-class of verbs


Open Class Words: Adjectives

Adjectives – denote qualities or properties of objects, e.g. heavy, blue, content

most languages have concepts for

colour - white, green, ...age - young, old, ...value- good, bad, ...

not all languages have adjectives as separate class


Open Class Words: Adverbs

Adverbs – denote modifications of actions (verbs), qualities (adjectives), e.g. walk slowly, heavily drunk

Directional or Locational AdverbsSpecify direction or location e.g. go home, stay here

Degree AdverbsSpecify extent of process, action, property e.g. extremely slow, very modest


Open Class Words: Adverbs 2

Manner AdverbsSpecify manner of action or process e.g. walk slowly, run fast

Temporal AdverbsSpecify time of event or action e.g. yesterday, Monday


Closed Word Classes

Closed Class Types:

prepositions: on, under, over, at, from, to, with, ...

determiners: a, an, the, ...

pronouns: he, she, it, his, her, who, I, ...

conjunctions: and, or, as, if, when, ...

auxiliary verbs: can, may, should, are

particles: up, down, on, off, in, out,

numerals: one, two, three, ..., first, second, ...


Closed Word Class: Prepositions

Prepositions

Occur before noun phrases

semantics: describe relations

often spatial or temporal relations


Closed Word Class: Pronouns

Pronouns Shorthand for referring to entity or eventSemantics: reference to entity

Personal Pronounsrefer to persons or entities, e.g. you, he, it, ...

Possessive Pronounspossession or relation between person and object, e.g.

his, her, my, its, ...

Wh-Pronounsreference in question or back reference, e.g.

Who did this ... Frieda, who is 80 years old ...


Closed Word Class: Conjunctions

Conjunctions Join two phrases, clauses, or sentencesSubordinating conjunction for embedded phrasesSemantics: difficult

Coordinating Conjunctionand, or, but, ...e.g. He takes the cat and the dog.

He takes the dog and she takes the dog.

Subordinating Conjunctionthat, ...e.g. He thinks that the cat is nicer than the dog.


Closed Word Class: Auxiliary Verbs

Auxiliary Verbs Mark semantic features of main verb Semantics: difficultTenseaddition expressing present, past or future, ...e.g. He will take the cat home.Aspectaddition expressing completion of actione.g. He is taking the cat home. (incomplete)Moodaddition expressing whether action is necessary ..e.g. He can take the cat home. (possible)


Closed Word Class: Copula

Copula and Modal Verbs subclass of Auxiliary Verbs State or tense description or modality of action Semantics: difficult (e.g. modal logic)

State / Process: be and do e.g. He is at home. He does nothing.

Tense: havee.g. He has taken the cat home.

Modality: can, ought to, should, muste.g. He can take the cat home. (possibility)


POS Tagging - Tagsets

Tagsets for English Penn Treebank, 45 tags Brown corpus, 87 tags C5 tagset, 61 tags C7 tagset, 146 tags

For references see textbook, p.296C5 and C7 tagsets are listed in textbook, Appendix C


POS Tagging - Taggers

Problems in POS Tagging:Ambiguity

Methods:Rule-Based Tagging

input is string of words, output is tagged stringStochastic Tagging

determines tags based on the probability of the occurrence of the tag, given the observed word, in the context of the preceding tags.

Christel Kemke

Sentence Level Constructs


Sentence Level Constructs I

Sentence Level Constructs I

declarative“This flight leaves at 9 am.”

S → NP VP

imperative“Book this flight for me.”

S → VP


Sentence Level Constructs II

Sentence Level Constructs II

yes-no-question“Does this flight leave at 9 am?”

S → Aux NP VP

wh-question “When does this flight leave Winnipeg?”

S → Wh-NP Aux NP VP


Noun Phrase Modification 1

Noun Phrase Modifiershead = the central noun of the NP modifiers = additions to head noun included in

NP modifiers before the head noun (prenominal) modifiers after the head noun (post-nominal)

examples: determiners, adjectives, PPs

e.g. the young man

the girl with the red hat


Noun Phrase Modification - Prenominal

determinerthe, a, this, some, ...

predeterminerall the flights

cardinal numbers, ordinal numbersone flight, the first flight, ...

quantifiersmuch, little


Noun Phrase Modification - Prenominal

adjectivesa first-class flight, a long flight

adjective phrasethe least expensive flight

Grammar RuleNP → (Det) (Card) (Ord) (Quant) (AP) Nominal


Noun Phrase Modification - Postnominal

prepositional phrase PPall flights from Chicago

Nominal → Nominal PP (PP) (PP) non-finite clause, gerundive postmodifers

all flights arriving after 7 pmNominal → GerundVPGerundVP → GerundV NP | GerundV PP | ...

relative clausea flight that serves breakfast

Nominal → Nominal RelClauseRelClause → (who | that) VP


Verb Subcategorization

Verb Phrase and SubategorizationVP = Verb + other constituents

(complements) Verb SubcategorizationDifferent verbs accept or need different

constituents or complements.Verbs can be classified according to the

complements they accept or need.


Verb Subcategorization and Complements

sentential complement

VP Verb inf-sentence

I want to fly from Boston to Chicago. NP complement

VP Verb NP

I want this flight.no complement

VP Verb I sleep.


Verb Subcategorization + Complements

more forms

VP Verb PP PP

I fly from Boston to Chicago.

Documents

Christel Kemke 2007/08 COMP 4060 Natural Language Processing Word Classes and English Grammar