74.793 nlp and speech 2004 english grammar describing natural language syntax: word classes and...
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74.793 NLP and Speech 2004English Grammar
Describing Natural Language Syntax: Word Classes and English Grammar (Ch.8 and 9)
• Word Classes / Part-of-Speech Tagging
• CFG with Grammatical Extensions• Sentence Structures• Noun Phrase - Modifications• Verb Phrase - Subcategorization
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Word Classes and POS Tagging
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Word ClassesSort 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
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Open vs. Closed Word Classes
Open Class Types
The 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
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Open vs. Closed Word Classes
Closed Class Types
The 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
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Open Class Words: NounsNouns – denote objects, concepts, …
Proper NounsNames for specific individual objects, entitiese.g. the Eiffel Tower, Dr. Kemke
Common NounsNames for categories or classes or abstractse.g. fruit, banana, table, freedom, sleep, ...
Count Nounsenumerable entities, e.g. two bananas
Mass Nounsnot countable items, e.g. water, salt, freedom
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Open Class Words: VerbsVerbs – denote actions, processes, states e.g. smoke, dream, rest, run
several morphological forms e.g.
non-3rd person - eat
3rd person - eats
progressive/ - eating present participle/ gerundive
past participle - eaten
Auxiliaries, e.g. be, as sub-class of verbs
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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
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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
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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
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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, ...
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Closed Word Class: Prepositions
Prepositions
Occur before noun phrases
semantics: describe relations
often spatial or temporal relations
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Closed Word Class: PronounsPronouns 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 ...
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Closed Word Class: ConjunctionsConjunctions 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.
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Closed Word Class: Auxiliary VerbsAuxiliary Verbs Mark semantic features of main verb Semantics: difficult
Tenseaddition 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)
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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: have
e.g. He has taken the cat home.
Modality: can, ought to, should, must
e.g. He can take the cat home. (possibility)
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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.296
C5 and C7 tagsets are listed in textbook, Appendix C
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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.
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Sentence Level Constructs
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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
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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
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Noun Phrase Modification 1
Noun Phrase Modifiers
head = 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
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Noun Phrase Modification - Prenominal
determinerthe, a, this, some, ...
predeterminerall the flights
cardinal numbers, ordinal numbersone flight, the first flight, ...
quantifiersmuch, little
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Noun Phrase Modification - Prenominal
adjectives
a first-class flight, a long flight
adjective phrase
the least expensive flight
Grammar Rule
NP → (Det) (Card) (Ord) (Quant) (AP) Nominal
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Noun Phrase Modification - Postnominal prepositional phrase PP
all flights from Chicago
Nominal → Nominal PP (PP) (PP)
non-finite clause, gerundive postmodifers
all flights arriving after 7 pm
Nominal → GerundVPGerundVP → GerundV NP | GerundV PP
| ...
relative clause
a flight that serves breakfast
Nominal → Nominal RelClauseRelClause → (who | that) VP
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Verb Subcategorization
Verb Phrase and Subategorization
VP = Verb + other constituents (complements)
Verb Subcategorization
Different verbs accept or need different constituents or complements.
Verbs can be classified according to the complements they accept or need.
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Verb Subcategorization + 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.
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Verb Subcategorization + Complements
more forms
VP Verb PP PP
I fly from Boston to Chicago.