Context-Free Grammars for English 1 인공지능 연구실 허 희 근

Context-Free Grammars for English 2 Contents  Introduce  What is a CFG (Context-Free Grammar)  Context-Free Rules and Trees  Sentence-Level Constructions  The Noun Phrase  Coordination  Agreement  The Verb Phrase and Subcategorization

Context-Free Grammars for English 3 Introduce(1)  Introduce a number of more complex notion of syntax and grammar. –Three main new ideas Constituency : 단일 unit 또는 phrase 를 가지는 word group –Ex) noun pharse Grammatical relations : SUBJECTS 와 OBJECTS 에 대한 전통문법 형식화 –Ex) She ate a mammoth breakfast. »she (SUBJECT), a mammoth breakfast (OBJECT) Subcategorization and dependencies : words 와 phrase 사이의 relation

Context-Free Grammars for English 4 Introduce(2)  Introduce the use of Context-Free Grammars. –All of these kinds of syntactic knowledge can be modeled by various kinds of grammars that are based on CFG. –CFG are backbone of many models of the syntax of natural language (of computer languages). –Powerful enough to express sophisticated relations among the words in a sentence. –CFG are integral to most models of natural language understanding, of grammar checking.

Context-Free Grammars for English 5 What is a CFG (Context-Free Grammar) (1)  영문에서 문장은 phrase 가 모여 이루어진 것이므로 문 장의 구조를 명확히 하기 위해서는 구의 구성을 아는 것이 기본이라는 것으로 출발  문장의 처음부터 끝까지 rewriting rule 을 적용함으로써 그 문장을 표현하는 parse tree 를 구성해 나가는 것

Context-Free Grammars for English 6 What is a CFG (Context-Free Grammar) (2)  Rewriting rule – 이러한 법칙을 문장의 처음부터 끝까지 적용하여 tree 를 만듦 (a) SNP·VP·PP (b) VP V·NP (c) NP DET·N (d) NP N (e) PP PREP·NP(f) N boy, window, hammer (g) DET the, a(h) V broke (i) PREP with

Context-Free Grammars for English 7 What is a CFG (Context-Free Grammar) (3)  CFG parse tree ( The boy broke the window with a hammer.)

Context-Free Grammars for English 8 Context-Free Rules and Trees(1)  CFG 는 일반적으로 영문이나 다른 자연어에서 그것을 구성하는 구조를 모델화할 때 가장 많이 쓰이는 수학적 시스템  CFG 는 rules 또는 productions 의 set 을 구성 – 이것들은 Language symbols 이 그룹화하고 순서를 정할 수 있 는 방법을 표현 –Symbols 는 CFG 가 두 개의 클래스로 나눌 때 사용 Terminal symbols : word 에 해당하는 symbols (“the”, “nightclub”) Non-terminals : terminal symbols 의 clusters 또는 generalization 을 표 현하는 symbols

Context-Free Grammars for English 9 Context-Free Rules and Trees(2)  CFG 는 보통 두 가지의 방법으로 생각함 – 문장을 일반화 시키기 위한 방법 – 주어진 문장의 구조의 정하기 위한 방법 –Generator : (arrow) Rewrite the symbol on the left with the string of symbols on the right. –CFG 는 랜덤하게 연속된 string 을 일반화시킬 때 사용 This sequence of rule expansions is called a derivation of the string of words. Represent a derivation by a parse tree

Context-Free Grammars for English 10 Context-Free Rules and Trees(3) Sample lexiconGrammar rule

Context-Free Grammars for English 11 Context-Free Rules and Trees(3) Parse tree

Context-Free Grammars for English 12 Sentence-Level Constructions  There are a great number of possible overall sentence structures. –But four are particularly common and important. Declarative structure Imperative structure Yes-no-question structure Wh-question structure

Context-Free Grammars for English 13 The Noun Phrase(1)  The syntax of English allows for both prenominal (pre-head) modifiers and post-nominal (post-head) modifiers. –Before the Head Noun Noun phrases can begin with a determiner –Ex) a stop, the flights, that fare, this flight, those flights… –Noun 이 복수형일 때는 determiner 생략할 수 있음 Predeterminers : word class appear in the NP before the determiner. –Common predeterminer is all –Ex) all the flights, all flight Word class appear in the NP between the determiner and the head noun. –Cardinal numbers, ordinal numbers, quantifiers rule of prenominal modifiers –NP (Det) (Card) (Ord) (Quant) (AP) Nominal

Context-Free Grammars for English 14 The Noun Phrase(2) –After the Noun A head noun can be followed by postmodifiers. –Prepositional phrases, relative clauses brackets inserted to show the boundaries of each PP. –Ex) a reservation [on flight six oh six] [form Tampa] [to Montreal] –Nominal Nominal PP (PP) (PP)

Context-Free Grammars for English 15 Coordination  Noun phrases and other units can be conjoined with conjunctions like and, or, and but. –NP NP and NP –VP VP and VP –S S and S

Context-Free Grammars for English 16 Agreement  Rule of 3sg subjects and non-3sg subjects –S Aux NP VP S 3sgAux 3sgNP VP S Non3sgAux Non3sgNP VP –3SgNP (Det) (Card) (Ord) (Quant) (AP) SgNominal –Non3SgNP (Det) (Card) (Ord) (Quant) (AP) PlNominal 3sgAux : does | has | can | … Non3sgAux : do | have | can | …

Context-Free Grammars for English 17 The Verb Phrase and Subcategorization Some subcategorization frames and example verbs