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An Introduction to Natural Language Syntax

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  1. An Introduction to Natural Language Syntax Rajat Mohanty rkm@cse.iitb.ac.in CS-460/IT-632 Department of Computer Science and Engineering Indian Institute of Technology, Bombay

  2. Outline • Grammatical Analysis • Finite State Grammar • Phrase Structure Grammar • Transformational Grammar • Natural Language Phenomena

  3. A Ubiquitous Task for NLP • Sequence labeling task can be at different levels. • In written text • Words • Phrases • Sentences • Paragraphs

  4. Names for Labeling Tasks • Words: Part of Speech tagging • Phrases: Chunking • Sentences: Parsing • Paragraphs: Co-reference annotating

  5. Example (Words: POS Tagging) <s> The dispute shows clearly the global power of Japan's financial titans.</s> <s>[ The/DT dispute/NN ] shows/VBZ clearly/RB [ the/DT global/JJ power/NN ] of/IN [ Japan/NNP 's/POS financial/JJ titans/NNS ] ./. </s>

  6. Example (Phrases: Chunking) The dispute shows clearly the global power of Japan's financial titans

  7. Example (Sentences: Parsing) ( (S (NP-SBJ The dispute) (VP shows (ADVP-MNR clearly) (NP (NP the global power) (PP of (NP (NP Japan 's) financial titans)))) .))

  8. Parse Tree S NP VP Det NP V NP N Det JJ N PP The dispute the global power shows of Japan’s financial titans

  9. Example (Sentences: Co-referencing) ( (S (NP-SBJ-1 The banks) (VP (ADVP-MNR badly) want (S (NP-SBJ *-1) (VP to (VP break (PP into (NP (NP all aspects) (PP of (NP the securities business))))))))

  10. What is Grammar? • A theory of language • A theory of competence of a native speaker (in the context of a Natural Language) • A finite set of rules • that generates only and all sentences of a language. • that assigns an appropriate structural description to each one. • An explicit model of competence

  11. What are the requirements? • An explicit model of competence • Should be able to generate an infinite set of grammatical sentences of the language • Should not generate any ungrammatical ones • Should be able to account for ambiguities (i.e., If a sentence is understood to have two meanings, the grammar should give two different structural description) • If two sentences are understood to have same meaning, the grammar should give the same structure for both at some level • If two sentences are understood to have different internal relationship, the grammar should assign different structural description

  12. What is Syntax? • Syntax is the study of the combination of words into phrases, clauses and sentences • Syntax describes how sentences and their constituents are structured

  13. Grammatical Analysis Techniques • Two main devices Labeling the Constituents Breaking up a String • Sequential • Hierarchical • Transformational • Morphological • Categorial • Functional • A grammar may combine any of these devices for grammatical analysis.

  14. Breaking up and Labeling • Sequential Breaking up • Sequential Breaking up and Morphological Labeling • Sequential Breaking up and Categorial Labeling • Sequential Breaking up and Functional Labeling • Hierarchical Breaking up • Hierarchical Breaking up and Categorial Labeling • Hierarchical Breaking up and Functional Labeling

  15. Sequential Breaking up • That student solved the problems. student that + + solve + + + + s ed the problem

  16. Sequential Breaking up and Morphological Labeling • That student solved the problems. student that solve s ed the problem word word stem affix affix word stem

  17. Sequential Breaking up and Categorial Labeling • This boy can solve the problem. boy this can solve the problem N Det Aux Det V N • They called her a taxi. They call ed her a taxi Pron V Pron Det N Affix

  18. Sequential Breaking up and Functional Labeling They called her a taxi Subject Verbal Direct Object Indirect Object her a taxi They called Indirect Object Direct Object Subject Verbal

  19. Hierarchical Breaking up • Old men and women Old men and women Old men and women men and women and women Old men Old men and women Old men

  20. Hierarchical Breaking up and Categorial Labeling • Poor John ran away. S NP VP A N V Adv Poor John ran away

  21. Hierarchical Breaking up and Functional Labeling • Immediate Constituent (IC) Analysis • Construction types in terms of the function of the constituents: • Predication (subject + predicate) • Modification (modifier + head) • Complementation (verbal + complement) • Subordination (subordinator + dependent unit) • Coordination (independent unit + coordinator)

  22. Predication • [Birds]subject [fly]predicate S Subject Predicate Birds fly

  23. Modification • [A]modifier [flower]head • John [slept]head [in the room]modifier S Subject Predicate John Head Modifier In the room slept

  24. Complementation • He [saw]verbal [a lake]complement S Subject Predicate He Verbal Complement saw alake

  25. Subordination • John slept [in]subordinator [the room]dependentunit S Subject Predicate John Head Modifier Subordinator Dependent Unit slept in theroom

  26. Coordination • [John came in time] independent unit [but]coordinator [Mary was not ready] independent unit S Independent Unit Independent Unit Coordinator John came in time but Mary was not ready

  27. An Example • In the morning, the sky looked much brighter. S Modifier Head Subject Predicate Subordinator DU Modifier Modifier Head Head Verbal Complement Modifier Head In the morning, the sky looked much brighter

  28. Hierarchical Breaking up and Categorial / Functional Labeling • Hierarchical Breaking up coupled with Categorial /Functional Labeling is a very powerful device. • But there are ambiguities which demand something more powerful. • E.g., Love of God • Someone loves God • God loves someone

  29. Hierarchical Breaking up Categorial Labeling Functional Labeling Love of God Love of God Head Modifier Noun Phrase Prepositional Phrase Sub DU love of God love of God

  30. Types of Generative Grammar • Finite State Model (sequential) • Phrase Structure Model (sequential + hierarchical) + (categorial) • Transformational Model (sequential + hierarchical + transformational) + (categorial + functional)

  31. Finite State Model COMES MAN The machine begins in the initial state, runs through a sequence of states (producing a word with each transition), and ends in the final state (producing a sentence) THE MEN COME OLD MAN COMES THE MEN COME

  32. Phrase Structure Model

  33. Phrase Structure Grammar (PSG) A phrase-structure grammar G consists of a four tuple (V, T, S, P), where • V is a finite set of alphabets (or vocabulary) • E.g., N, V, A, Adv, P, NP, VP, AP, AdvP, PP, student, sing, etc. • T is a finite set of terminal symbols: T  V • E.g., student, sing,etc. • S is a distinguished non-terminal symbol, also called start symbol: S  V • P is a set of production rules

  34. Noun Phrases • John • the student • the intelligent student NP NP NP N N N Det Det AdjP student John student the the intelligent

  35. Noun Phrase • his first five PhD students NP N Quant Det Ord N students five his first PhD

  36. Noun Phrase • The five best students of my class NP PP Quant Det AP N five the best students of my class

  37. Verb Phrases • can sing • can hit the ball VP VP V Aux NP Aux V can sing the ball can hit

  38. Verb Phrase • Can give a flower to Mary VP NP Aux V PP a flower can give to Mary

  39. Verb Phrase • may make John the chairman VP NP Aux V NP John may make thechairman

  40. Verb Phrase • may find the book very interesting VP NP Aux V AP veryinteresting thebook may find

  41. Prepositional Phrases • in the classroom • near the river PP PP NP NP P P in near theclassroom theriver

  42. Adjective Phrases • intelligent • very honest • fond of sweets AP AP AP A PP Degree A A very fond honest ofsweets intelligent

  43. Adjective Phrase • very worried that she might have done badly in the assignment AP Degree A S’ very worried that she might have done badly in the assignment

  44. Phrase Structure Rules • The boy hit the ball. • Rewrite Rules: • S  NP VP • NP  Det N • VP  V NP • Det  the • N  boy, ball • V  hit • We interpret each rule X Yas the instruction rewrite X as Y.

  45. Derivation • The boy hit the ball. • Sentence NP + VP(1) S NP VP Det + N + VP (2) NP  Det N Det + N + V + NP(3) VP  V NP The + N + V + NP (4) Det  the The + boy + V + NP (5) N  boy The + boy + hit + NP (6) V  hit The + boy + hit + Det + N (2) NP  Det N The + boy + hit + the + N (4) Det  the The + boy + hit + the + ball(5) N  ball

  46. PSG Parse Tree • The boy hit the ball. S NP VP Det N V NP the Det N boy hit the ball

  47. PSG Parse Tree • John wrote those words in the Book of Proverbs. S NP VP PropN PP V NP NP P NP PP John in thosewords wrote ofproverbs thebook

  48. Transformational Model

  49. Transformational Grammar If a generative grammar makes use of all the three • Sequential • Hierarchical • transformational breaking up and two • categorial • functional labeling is called a Transformational grammar (Universal Grammar).

  50. Other Grammar Formalisms • Lexical Functional Grammar (LFG) • Generalised Phrase Structure Grammar (GPSG) • Tree Adjoining Grammar (TAG) • Categorial Grammar (CG) • Head-driven Phrase Structure Grammar (HPSG) • Systemic Functional Grammar (SFG)