1. Syntax
Sudeshna Sarkar
25 Aug 2008

2. Some Fundamental Questions
What is Language?
How to define a Language?
What makes a language different from another?
Is there anything common to all languages?

3. Syntax
Syntax: from Greek syntaxis, “setting out together, arrangmenet’
Refers to the way words are arranged together, and the relationship between them.
Distinction:
Prescriptive grammar: how people ought to talk
Descriptive grammar: how they do talk
Goal of syntax is to model the knowledge of that people unconsciously have about the grammar of their native language

4. The Two Schools Rationalists Empiricists
It’s all hardcoded in our brains
Principle and Parameter Theory
Poverty of Stimulus
Recursion
Empiricists
Just a special kind of pattern recognition
No different from other cognitive abilities like vision
Language is a stochastic phenomenon

5. The Generative Grammar
“The grammatical principles underlying languages are innate and fixed, and the differences among the world's languages can be characterized in terms of parameter settings in the brain …” -
Noam Chomsky [1928-]
Courtesy

6. I & E Languages
I – Language: Mentally represented system of rules (I – internal)
E – Language: Observable external products of I-language (written text, utterances)
Language: Collective E-language of a very large group of speakers
Syntax: Study of the I-language from E-language

7. The Chomsky Hierarchy Grammar Languages Automaton Production rules
Type-0
Recursively enumerable
Turing machine
No restrictions
Type-1
Context-sensitive
Linear-bounded non-deterministic Turing machine
αAβ → αγβ
Type-2
Context-free
Non-deterministic pushdown automaton
A → γ
Type-3
Regular
Finite state automaton
A → aB A → a

8. From Formal to Natural Languages
Organizational Unit
Complexity
Word
Regular
Sounds
Sentence
Context-free
Discourse ??

9. Some Observations on NLs
Constituency: A group of words acts as a single unit – phrases, clauses etc.
Grammatical Relations: Different words/ phrases are related to the main verb of the sentence – object, subject, instrument
Subcategorization and Dependency Relations: Not all verbs can take all type of arguments – transitive, intransitive etc.

10. Syntax Why should you care? Grammar checkers Question answering
Information extraction
Machine translation

11. Why NLP is difficult: Newspaper headlines
Iraqi Head Seeks Arms
Juvenile Court to Try Shooting Defendant
Teacher Strikes Idle Kids
Stolen Painting Found by Tree
Local High School Dropouts Cut in Half
Red Tape Holds Up New Bridges
Clinton Wins on Budget, but More Lies Ahead
Hospitals Are Sued by 7 Foot Doctors
Kids Make Nutritious Snacks

12. Why is NLU difficult? The hidden structure of language is hugely ambiguous
Tree for: Fed raises interest rates 0.5% in effort to control inflation (NYT headline 5/17/00)

13. Where are the ambiguities?

14. The bad effects of V/N ambiguities

15. Context-Free Grammars
Capture constituency and ordering
Ordering is easy
What are the rules that govern the ordering of words and bigger units in the language
What’s constituency?
How words group into units and how the various kinds of units behave wrt one another

16. Constituency We have NLP classes from 5:30 to 6:30 pm on Tuesday.
On Tuesday we have NLP classes from 5:30 – 6:30 pm.
From 5:30 to 6:30 pm on Tuesday we have NLP classes.
We have NLP on Tuesday from 5:30 to 6:30 pm classes.
On we have NLP classes from Tuesday 5:30 to 6:30 pm.
From 5:30 we have to 6:30 pm on Tuesday NLP classes.

17. Constituency We have NLP classes from 5:30 to 6:30 pm on Tuesday.
On Tuesday we have NLP classes from 5:30 – 6:30 pm.
From 5:30 to 6:30 pm on Tuesday we have NLP classes.
We have NLP on Tuesday from 5:30 to 6:30 pm classes.
On we have NLP classes from Tuesday 5:30 to 6:30 pm.
From 5:30 we have to 6:30 pm on Tuesday NLP classes.

18. Phrases Phrase: Group of words that act as a unit Noun Phrase NP
A midsummer night’s dream, My experiments with truth, The man who knew infinity
Verb Phrase VP
Gone with the wind, Saving private Ryan
Prepositional Phrases PP
Of sons and lovers, to sir with love, Beyond the blue mountains, Into the heart of the mind

19. Modelling the Syntax of English
Let us try CFGs
S  NP VP I love India.
S  VP Love your country.
S  Aux NP VP Do you love your country?
S  Wh-NP VP Who loves his country?
S  Wh-NP Aux NP VP
Which country do you live in?

20. Phrase Structure Grammar
Context Free Grammars are also called phrase structure grammars
Phrases are the building blocks of any PSG (i.e. CFG)
Phrases in turn are defined by CFG (PSG)

21. I think that Einstein thought that Newton said …
Is CFG Necessary?
Can we model the syntax of English using Regular Grammar?
NO! we cannot model recursion in RG
S  NP VP
VP  Verb S
I think that Einstein thought that Newton said …

22. CFG Examples S -> NP VP NP -> Det NOMINAL NOMINAL -> Noun
VP -> Verb
Det -> a
Noun -> flight
Verb -> left

23. CFGs
S -> NP VP
This says that there are units called S, NP, and VP in this language
That an S consists of an NP followed immediately by a VP
Doesn’t say that that’s the only kind of S
Nor does it say that this is the only place that NPs and VPs occur

24. Context Free Grammars A CFG consists of a tuple (N,T,S,P)
N is a finite set of non-terminal symbols
T is a finite set of terminal symbols
S is the start symbol
P is a finite set of rules of the form X   where X  N and {N U T}*

25. Phrase Structure Parsing
Phrase structure organizes words into phrases, often called constituents
This organization is hierarchical
For a given string there is often ambiguity about the correct phrase structure
This ambiguity often corresponds to semantic ambiguity

27. Simple examples of a CFG
Take the non-terminals = {S, NP, VP, V}
And the terminals {boys, study, play, books, cricket)
Let the start symbol be S
Let the rule set be
S  NP VP
VP  V
VP  V NP
NP  boys
NP  books
NP  cricket
V study
V play
This CFG licenses a finite number of tree sentences

28. Generativity
As with FSAs and FSTs you can view these rules as either analysis or synthesis machines
Generate strings in the language
Reject strings not in the language
Impose structures (trees) on strings in the language

29. Derivations
A derivation is a sequence of rules applied to a string that accounts for that string
Covers all the elements in the string
Covers only the elements in the string

30. Derivations as Trees

31. Two views of linguistic structure: 1. Constituency (phrase structure)
Phrase structure organizes words into nested constituents.
How do we know what is a constituent? (Not that linguists don't argue about some cases.)
Distribution: a constituent behaves as a unit that can appear in different places:
John talked [to the children] [about drugs].
John talked [about drugs] [to the children].
*John talked drugs to the children about
Substitution/expansion/pro-forms:
I sat [on the box/right on top of the box/there].
Coordination, regular internal structure, no intrusion, fragments, semantics, …

32. Two views of linguistic structure: 2. Dependency structure
Dependency structure shows which words depend on (modify or are arguments of) which other words.
put
boy
tortoise on
rug
The
the
The
boy
put
the
tortoise on
the
rug
the

33. Parsing
Parsing is the process of taking a string and a grammar and returning a (many?) parse tree(s) for that string
It is completely analogous to running a finite-state transducer with a tape
It’s just more powerful
Remember this means that there are languages we can capture with CFGs that we can’t capture with finite-state methods

34. Other Options Regular languages (expressions)
Too weak
Context-sensitive or Turing equiv
Too powerful (maybe)

35. Context?
The notion of context in CFGs has nothing to do with the ordinary meaning of the word context in language.
All it really means is that the non-terminal on the left-hand side of a rule is out there all by itself (free of context)
A -> B C
Means that
I can rewrite an A as a B followed by a C regardless of the context in which A is found
Or when I see a B followed by a C I can infer an A regardless of the surrounding context

36. Key Constituents (English)
Sentences
Noun phrases
Verb phrases
Prepositional phrases