Morphology Words and their Parts CS 4705 Basic

Morphology: Words and their Parts CS 4705

Basic Uses of Morphology • The study of how words are composed from smaller, meaning-bearing units (morphemes) • Applications: – Spelling correction: referece – Hyphenation algorithms: refer-ence – Part-of-speech analysis: googler – Text-to-speech: grapheme-to-phoneme conversion • hothouse (/T/ or /D/)

– Speech recognition: phoneme-to-grapheme conversion – Amusing poetry and artificial languages in standardized tests • ‘Twas brillig and the slithy toves… • Muggles moogled migwiches

What is a word? • In formal languages, words are arbitrary strings • In natural languages, words are made up of meaningful subunits called morphemes – Allows for productivity: googled, texted – Abstract concepts denoting entities or relationships in the world • Roots + • Syntactic or grammatical elements – Realizations of morphemes: morphs • Door realizes door; take and took realize take

• Allomorphs are classes of related morphs that realize a given morpheme – Allomorphs of s include en, men, es in English – Take and took are allomorphs of take – Sum: Morpheme [s] is realized by an allomorph class that includes the related morphs {en, men, es} – Syntactic or grammatical morphemes can convey many things – In Italian, mark nouns for gender and number Singular Plural Masc pomodoro pomodori Fem cipolla cipolle pomodor- cipoll-: stems, may or may not occur on their own as words – Stem may not occur as a word: derivative/deriv – Base form (lemma) occurs as word: derivative/derive – Sometimes the same: cars has stem ‘car’ and base form or lemma ‘car’ too

What useful information does morphology give us? • Different things in different languages – Spanish: hablo, hablaré/ English: I speak, I will speak – English: book, books/ Japanese: hon, hon • Languages differ in how they encode morphological information – Isolating languages (e. g. Cantonese) have no affixes: each word usually has 1 morpheme – Agglutinative languages (e. g. Finnish, Turkish) are composed of prefixes and suffixes added to a stem (like beads on a string) – each feature realized by a single affix, e. g. Finnish

epäjärjestelmällistyttämättömyydellänsäkäänköhän ‘Wonder if he can also. . . with his capability of not causing things to be unsystematic’ – Inflectional languages (e. g. English) merge different features into a single affix (e. g. ‘s’ in likes indicates both person and tense); and the same feature can be realized by different affixes – Polysynthetic languages (e. g. Inuit languages) express much of their syntax in their morphology, incorporating a verb’s arguments into the verb, e. g. Western Greenlandic Aliikusersuillammassuaanerartassagaluarpaalli. aliiku-sersu-i-llammas-sua-a-nerar-ta-ssa-galuar-paal-li entertainment-provide-SEMITRANS-one. good. at-COP-say. that-REPFUT-sure. but-3. PL. SUBJ/3 SG. OBJ-but 'However, they will say that he is a great entertainer, but. . . ' – So…. different languages may require very different morphological analyzers

Morphology Can Help Define Word Classes • AKA morphological classes, parts-of-speech • Closed vs. open (function vs. content) class words – Pronoun, preposition, conjunction, determiner, … – Noun, verb, adjective, … • Identifying word classes is useful for almost any task in NLP, from translation to speech recognition to topic detection…very basic semantics

(English) Inflectional Morphology Word stem + grammatical morpheme different forms of same word – Usually produces word of same class – Usually serves a syntactic or grammatical function (e. g. agreement) likes or liked birds • Nominal morphology – Plural forms • s or es • Irregular forms (goose/geese)

• Mass vs. count nouns (fish/fish(es), email or emails? ) – Possessives (cat’s, cats’) • Verbal inflection – Main verbs (sleep, like, fear) relatively regular • -s, ing, ed • And productive: emailed, instant-messaged, faxed, homered • But some are not: – eat/ate/eaten, catch/caught – Primary (be, have, do) and modal verbs (can, will, must) often irregular and not productive » Be: am/is/are/were/was/been/being – Irregular verbs few (~250) but frequently occurring

• Particles occur in only one form: in English – Prepositions: to, from – Adverbs: happily, quickly – Conjunctions: but, and – Articles: the, a, an – Japanese? • So…. English inflectional morphology is fairly easy to model…. with some special cases. . .

Derivational Morphology • Word stem + syntactic/grammatical morpheme new words – Usually produces word of different class – Incomplete process: derivational morphs cannot be applied to just any member of a class • Verbs --> nouns – -ize verbs -ation nouns – generalize, realize generalization, realization – synthesize but no synthesization

• Verbs, nouns adjectives – embrace, pity embraceable, pitiable – care, wit careless, witless • Adjective adverb – happy happily • Process selective in unpredictable ways – Less productive: nerveless/*evidence-less, malleable/*sleep-able, rar-ity/*rareness – Meanings of derived terms harder to predict by rule • clueless, careless, nerveless, sleepless

• Derivation can be applied recursively: – Hospital hospitalize hospitalization prehospitalization … – Morphological analysis identifies concatenative processes as well as morphemes [pre[[[hospital]ize]ation]] – But there are bracketing paradoxes unhappier [un[happier]: not happier [[unhappy]er]: more unhappy

Compounding • Two base forms join to form a new word – Bedtime, Weinerschnitzel, Rotwein – Careful? Compound or derivation?

Affixes can be attached to stems in different ways – Prefixation • Immaterial – Suffixation: more common across languages than prefixation • Trying – Circumfixation: combine prefixation and suffixation • Gesagt

– Infixation • English: Absobl**dylutely • Bontoc: ‘um’ turns adjectives and nouns into verbs (kilad (red) kumilad (to be red))

Concatenative vs. Non-concatenative Morphology • Semitic root-and-pattern morphology – Root (2 -4 consonants) conveys basic semantics (e. g. Arabic /ktb/) – Vowel pattern conveys voice and aspect – Derivational template (binyan) identifies word class

Template CVCVC CVCCVC CVVCVC t. VCVVCVC n. CVVCVC Ct. VCVC st. VCCVC Vowel Pattern active passive katab kutib write kattab kuttib cause to write ka: tab ku: tib correspond taka: tab tuku: tib write each other nka: tab nku: tib subscribe ktatab ktutib write staktab stuktib dictate

Morphotactics • What are the ‘rules’ for constructing a word in a given language? – Pseudo-intellectual vs. *intellectual-pseudo – Rational-ize vs *ize-rational – Cretin-ous vs. *cretin-ly vs. *cretin-acious • Possible ‘rules’ – Suffixes are suffixes and prefixes are prefixes – Certain affixes attach to certain types of stems (nouns, verbs, etc. ) – Certain stems can/cannot take certain affixes

• Semantics: In English, un- cannot attach to adjectives that already have a negative connotation: – Unhappy vs. *unsad – Unhealthy vs. *unsick – Unclean vs. *undirty • Phonology: In English, -er cannot attach to words of more than two syllables – great, greater – Happy, happier – Competent, *competenter – Elegant, *eleganter – Unruly, ? unrulier

Morphological Parsing • These regularities enable us to create software to parse words into their component parts – Known words and new ones (e. g. Pneumonoultramicroscopicsilicovolcanoconiosi s, Columbianize, Columbianization)

Morphological Representations: Evidence from Human Performance • Hypotheses: – Full listing hypothesis: words listed – Minimum redundancy hypothesis: morphemes listed • Experimental evidence: – Priming experiments (Does seeing/hearing one word facilitate recognition of another? ) suggest neither – Regularly inflected forms (e. g. cars) prime stem (car) but not derived forms (e. g. management, manage)

– But spoken derived words can prime stems if they are semantically close (e. g. government/govern but not department/depart) • Speech errors suggest affixes must be represented separately in the mental lexicon – ‘easy enoughly’ for ‘easily enough’

Summing Up • Different languages have different morphological systems – If we can discover how to decode such a system, we can identify useful information about the word class and the semantic meaning of a word – Morphological regularities provide basis for building (automatic) morphological analyzers • Next time: Read Ch 3. 2 -3. 6 – HW 1 will be assigned (check the course syllabus and courseworks)

Announcements • HW 1 will now be due 9/25/07 • WICS lunch tomorrow at noon in the CS Lounge, 452 MUDD (rsvp to hila@cs. columbia. edu)