Grammars for a Natural Language

1
Grammars for a Natural Language

• 5.1 Auxiliary Verbs and Verb Phrases
• 5.2 Movement Phenomena in Language
• 5.3 Handling Questions in CFGs
• 5.4 Relative Clauses
• 5.5 The Hold Mechanism in ATNs
• 5.6 Gap Threading

2
5.1 Auxiliary Verbs

• Additional pre-verb completing sentence
• can, have, shall, may, do, will, must, need,
  dare, ought to, used to, be going to
• I will watch the movie.
• My friend should see it for himself.
• Henry might want to take a vacation.
• Next week, they will have been missing for
  fifteen days.
• Very ordered structure in grammar (not random)
• Auxiliary verbs determine main verb structure

3
Testing for Auxiliary Verbs

• Can be placed before not
• He can not say those things.
• I eat not a bite more.
• Can precede the subject NP in a question
• Would we run the entire race?
• Enjoys she her job?
• May not appear as the only verb
• John shall the weekend.
• Mary loves the weekend.

4
Modal COMPFORM

• Modal auxiliaries exclude be, have
• Appear only in finite tense (present and past)
• Can only be auxiliary verbs
• Non-modal auxiliaries include be, have
• Can appear in variety of tenses
• Can be main verbs as well as auxiliary
• COMPFORM
• Feature used to specify form of main verb

5
COMPFORM Chart

• Chart reproduced from Fig. 5.1 on p. 124 of text

• I can see the house. COMPFORM base
• I have seen the house. COMPFORM pastprt
• I am lifting the box. COMPFORM ing
• I was seen by the crowd. COMPFORM pastprt

6
Grammars with Auxiliaries

• Grammar rule for VP
• VP ? (AUX COMPFORM ?s)(VP VFORM ?s)
• I must complete this work.
• I have known that answer for a while.
• I am crying.
• Restriction 1
• Modal have be (progressive) be (passive)
• The song might have been being played as we
  left.

7
More Auxiliary Restrictions

• 1 Modal have be (prog.) be (pass.)
• Exception I regret having been chosen to go.
• 2 Auxiliary sequences can appear as VP
  complements for certain verbs
• Exception I must be having been singing.
• 3 Add a MAIN feature to signal a main verb, and
  a PASS feature to signal passive
• VP ? AUXbe VPing, main
• VPpass ? AUXbe VPpassprt, main

8
Features main, pass

• VP ? AUXbe VPing, main
• VPpass ? AUXbe VPpassprt, main
• VP ? V....
• Examples
• I was helping the boy.
• I was seen by the boy.
• I was being good.
• Parse trees at Fig. 5.4 on page 128 of text

9
Passive Sentences

• Passive sentences reverse order of NPs
• I will hide my hat.
• The hat will be hidden by me.
• Object may be removed from passive
• The hat will be hidden.
• Feature signals missing object passgap
• VPpassgap ? VPnp
• VP-passgap ? VPnp NP

10
5.2 Movement Phenomena

• Movement Phenomena moving parts of a sentence
  around, such as in questioning
• Subject-aux inversion
• Rearranging the subject and auxiliary verb to ask
  a question
• John is going to the store.
• Is John going to the store?
• Henry found four dollars.
• Did Henry find four dollars?

11
Local Unbounded Movement

• Local Movement
• Movement of constituents within limited rules
• Subject-aux inversion
• Unbounded Movement
• Arbitrary movement of constituents in sentence
• Wh-questions
• Wh-questions
• Forming a question by using a wh- word (e.g.,
  what, who, why, when)

12
Wh-questions

• Joe had written a letter to the congressman last
  night.
• Who had written a letter to the congressman
  last night?
• What had Joe written to the congressman last
  night?
• To whom had Joe written a letter last night?
• When had Joe written a letter to the congressman
  ?
• Inverts auxiliary and subject
• Allows inversion rules to apply
• Questions begin with a wh-word
• A gap appears in each sentence at

13
Handling Gaps

• Gap space where word has been removed
• Filler moved constituent
• Transformational Grammar
• Maps a regular CFG (deep structure) to an altered
  CFG (surface structure)
• Box 5.1 on page 131 of text
• Hold List on CFGs
• Allows constituents to be saved until later

14
Handling Gaps

• Slash categories
• Nonterminals of form X/Y, where X is the real
  constituent and Y is the missing constituent
• Increases grammar size
• Feature GAP
• Same format as slash category
• Grammar size not increased

15
Other Movement Types

• Box 5.2 on p. 132 of text
• Topicalization
• Movement for emphasis
• This picture, I had never liked.
• Adverb preposing
• Adverb moved to beginning of sentence
• Tomorrow, I will see you.
• Extraposition
• NP moved to sentence close
• A book was written discussing evolution

16
5.3 Handling Questions in CFGs

• Goal Extend CFG minimally for questions
• Yes/No questions
• Sinv ? (AUX COMPFORM ?s) (NP) (VP VFORM ?s)
• Gap Rule
• (NP GAP ((CAT NP)(AGR ?a)) AGR ?a) ? ?
• Nonlexical subconstituent
• (S GAP ?g) ? (NP GAP -)(VP GAP ?g)
• Lexical subconstituent
• (VP GAP ?g) ? V_np_vpinf(NP GAP ?g)(PP GAP-)
• (VP GAP ?g) ? V_np_vpinf(NP GAP -)(PP GAP ?g)

17
Analyzing Fillers

• Filler moved subconstituent
• Word types
• Pronouns who, whom, what
• Determiners what, which
• Prepositions where, when
• Adverbial modifier how
• Possessive Pronoun whose
• Feature WH
• Value Q wh-question
• Value R relative clause

18
New Syntactic Categories

• WH (feature, not syntactic category)
• Signals question
• Propagates up through rules (i.e., WH PP S)
• QDET
• Determiners introducing WH-terms
• PP-WRD
• Words that act as prepositional phrases
• CNP
• Common noun phrase

19
Grammar Rules for Wh-words

• Grammar 5.7 on p. 135 of the text
• 2. (NP WH ?w) ? (DET WH ?w AGR ?a)(CNP AGR ?a)
• 3. CNP ? N
• 5. DET ? ART
• 7. (DET WH ?w) ? (QDET WH ?w)
• 8. (PP WH ?w) ? P (NP WH ?w)
• 9. (PP WH ?w) ? (PP-WRD WH ?w)
• Examples
• Which book did you read? 2, 7, 3
• Where did you find the file? 9
• In what context did you mean that? 8, 2, 7, 3

20
Grammar Rules for Wh-questions

• Grammar 5.9 on p. 137 of text
• 11. (Sinv WH ?w GAP ?g) ? (AUX COMPFORM ?s AGR
  ?a)(NP WH ?w AGR ?a GAP-)(VP VFORM ?s GAP ?g)
• 12. S ? (NPQ,-gap AGR ?a)(Sinv GAP (NP AGR
  ?a))
• 16. (VP GAP ?g) ? V_np (NP GAP ?g)
• Example I will purchase the new book.
• What book will I purchase?
• 12 NP What book S will I purchase
• 2, 7, 3 QDET What CNP book
• 11 AUX will NP I VP purchase
• 16 V purchase NP ?

21
Parsing With Gaps

• Top-down parsing
• Not problematic gaps are predictable
• Bottom-up parsing
• Problematic gaps may exist at any position
• Strategy for charting
• Automatically insert gap wherever it is legal
• Works for both top-down and bottom-up, although
  top-down creates fewer combinations of
  constituents with gaps

22
Parsing Example With Gaps

• Which dogs did he see?
• Rule 7 Which Rule 3 dogs
• Rule 12 Which dogs
• Rule 11 did he see
• Rule 1 he
• Rule 16 see Add EMPTY NP
• No unnecessary EMPTY constituents added here

23
5.4 Relative Clauses

• Embedded sentence missing a noun, connected by a
  special relative pronoun (who, whom, which, when,
  where, whose)
• New syntactic category REL
• CNP ? CNP REL
• REL ? (NP WH R AGR ?a)(S-inv, fin GAP (NP AGR
  ?a))
• REL ? (PP WH R PFORM ?p)(S-inv, fin GAP (PP
  PFORM ?p))
• The man who we saw at the store was the culprit.
• The exam in which you found the error was
  corrected.
• The man whose book you stole is going to the
  police.

24
Gaps in Relative Clause

• Gaps are not allowed inside subject
• Subject of relative clause becomes relative
  pronoun
• REL ? NPR VPfin
• The firefighter who rescued the people
• The newspaper which ran the story
• The man who watched the baby

25
Relative Pronoun That

• Added to wh-words already listed
• WH feature set to value R only
• Can use all rules applied to R wh-words
• The book that I lent you
• The search that began four days ago
• Sometimes the relative pronoun is missing
  altogether (implied)
• The investigation (that) Mary has undertaken
• The programmers (who) I recommended

26
Reduced Relative Clauses

• Relative clauses without an explicit pronoun
  (implied) AND that implied pronoun is the missing
  subject
• The damage caused by the storm was awful.
• The issue creating the argument is age-old.
• Additional rules
• REL ? (VP VFORM ing pastprt)
• REL ? (Sfin GAP (NP AGR ?a))
• Although multiple gaps now can appear in a
  sentence, they do not conflict

27
5.5 Hold Mechanism in ATNs

• Hold list
• Alternative method for handling movement (to gap
  features)
• Functions by holding special constituents to be
  used instead at other places
• Hold list stores constituents until later
• Gaps process constituents as they exist, but plan
  for where they are missing

28
Holding

• Hold action
• Adds a constituent to a hold list
• Occurs on an arc transfer in an ATN
• HOLD
• Emptying list
• VIR arc removes held word of specified type
• Cannot pop from ATN until list is completely
  emptied

29
Holding Examples

• Grammar 5.14 on p. 145 of the text
• Example sentences
• The dog bit the cat.
• Can you find the car?
• Who ate the pizza?
• The man who ate the pizza spent fifteen dollars.
• Who do I see?

30
Comparing GAPs Hold Lists

• Three considerations
• Coverage Can it handle all examples?
• Selectivity Does it reject ill-formed examples?
• Conciseness How easily are the rules specified?
• Coverage is equal among the two methods
• Selectivity and conciseness need to be analyzed
  and considered

31
Selectivity Conciseness

• GAPs require an NP with an NP gap be empty
• ATNs allow held constituents to be used in ways
  that violate this rule
• Who did the man who saw hit the boy?
• Relative clause who saw
• Main sentence Who did the man hit the boy
• To make ATN as selective as GAPs, add complexity,
  reduce conciseness
• Fine line between formalism as enabler and
  restrictor

32
5.6 Gap Threading

• Third method for handling rearrangement of
  constituents
• Used in logic grammars
• Predicates defined containing
• Position-in and position-out, which define the
  range of the input
• A list of fillers that exist before the rule is
  applied
• A list of fillers existing after the rule is
  applied

33
Filler Lists

• Subconstituents can be removed from the input
  filler list if they are used as part of the
  constituents construction
• Subconstituents can be added to the output filler
  list if they are parsed but not used as part of
  the constituent
• Flexible, but constraints are explicitly
  enforced, not part of the formalism (similar to
  hold lists)