Machine Translation Challenges and Language Divergences

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Machine TranslationChallenges and Language
Divergences

• Alon Lavie
• Language Technologies Institute
• Carnegie Mellon University
• 11-731 Machine Translation
• January 12, 2011

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Major Sources of Translation Problems

• Lexical Differences
• Multiple possible translations for SL word, or
  difficulties expressing SL word meaning in a
  single TL word
• Structural Differences
• Syntax of SL is different than syntax of the TL
  word order, sentence and constituent structure
• Differences in Mappings of Syntax to Semantics
• Meaning in TL is conveyed using a different
  syntactic structure than in the SL
• Idioms and Constructions

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Lexical Differences

• SL word has several different meanings, that
  translate differently into TL
• Ex financial bank vs. river bank
• Lexical Gaps SL word reflects a unique meaning
  that cannot be expressed by a single word in TL
• Ex English snub doesnt have a corresponding
  verb in French or German
• TL has finer distinctions than SL ? SL word
  should be translated differently in different
  contexts
• Ex English wall can be German wand (internal),
  mauer (external)

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Google at Work
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Lexical Differences

• Lexical gaps
• Examples these have no direct equivalent in
  Englishgratiner(v., French, to cook with a
  cheese coating)otosanrin(n., Japanese,
  three-wheeled truck or van)

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Lexical Differences
From Hutchins Somers
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MT Handling of Lexical Differences

• Direct MT and Syntactic Transfer
• Lexical Transfer stage uses bilingual lexicon
• SL word can have multiple translation entries,
  possibly augmented with disambiguation features
  or probabilities
• Lexical Transfer can involve use of limited
  context (on SL side, TL side, or both)
• Lexical Gaps can partly be addressed via phrasal
  lexicons
• Semantic Transfer
• Ambiguity of SL word must be resolved during
  analysis ? correct symbolic representation at
  semantic level
• TL Generation must select appropriate word or
  structure for correctly conveying the concept in
  TL

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Structural Differences

• Syntax of SL is different than syntax of the TL
• Word order within constituents
• English NPs art adj n the big boy
• Hebrew NPs art n art adj ha yeled ha gadol
• Constituent structure
• English is SVO Subj Verb Obj I saw the man
• Modern Arabic is VSO Verb Subj Obj
• Different verb syntax
• Verb complexes in English vs. in German
• I can eat the apple Ich kann den apfel essen
• Case marking and free constituent order
• German and other languages that mark case
• den apfel esse Ich the(acc) apple eat I(nom)

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MT Handling of Structural Differences

• Direct MT Approaches
• No explicit treatment Phrasal Lexicons and
  sentence level matches or templates
• Syntactic Transfer
• Structural Transfer Grammars
• Trigger rule by matching against syntactic
  structure on SL side
• Rule specifies how to reorder and re-structure
  the syntactic constituents to reflect syntax of
  TL side
• Semantic Transfer
• SL Semantic Representation abstracts away from SL
  syntax to functional roles ? done during analysis
• TL Generation maps semantic structures to correct
  TL syntax

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Syntax-to-Semantics Differences

• Meaning in TL is conveyed using a different
  syntactic structure than in the SL
• Changes in verb and its arguments
• Passive constructions
• Motion verbs and state verbs
• Case creation and case absorption
• Main Distinction from Structural Differences
• Structural differences are mostly independent of
  lexical choices and their semantic meaning ?
  addressed by transfer rules that are syntactic in
  nature
• Syntax-to-semantic mapping differences are
  meaning-specific require the presence of
  specific words (and meanings) in the SL

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Syntax-to-Semantics Differences

• Structure-change example
• I like swimming
• Ich scwhimme gern
• I swim gladly

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Syntax-to-Semantics Differences

• Verb-argument example
• Jones likes the film.
• Le film plait à Jones.
• (lit the film pleases to Jones)
• Use of case roles can eliminate the need for this
  type of transfer
• Jones Experiencer
• film Theme

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Syntax-to-Semantics Differences

• Passive Constructions
• Example French reflexive passivesCes livres se
  lisent facilementThese books read themselves
  easilyThese books are easily read

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Same intention, different syntax

• rigly bitiwgacny
• my leg hurts
• candy wagac fE rigly
• I have pain in my leg
• rigly bitiClimny
• my leg hurts
• fE wagac fE rigly
• there is pain in my leg
• rigly bitinqaH calya
• my leg bothers on me
• Romanization of Arabic from CallHome Egypt.

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MT Handling of Syntax-to-Semantics Differences

• Direct MT Approaches
• No Explicit treatment Phrasal Lexicons and
  sentence level matches or templates
• Syntactic Transfer
• Lexicalized Structural Transfer Grammars
• Trigger rule by matching against lexicalized
  syntactic structure on SL side lexical and
  functional features
• Rule specifies how to reorder and re-structure
  the syntactic constituents to reflect syntax of
  TL side
• Semantic Transfer
• SL Semantic Representation abstracts away from SL
  syntax to functional roles ? done during analysis
• TL Generation maps semantic structures to correct
  TL syntax

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Idioms and Constructions

• Main Distinction meaning of whole is not
  directly compositional from meaning of its
  sub-parts ? no compositional translation
• Examples
• George is a bull in a china shop
• He kicked the bucket
• Can you please open the window?

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Formulaic Utterances

• Good night.
• tisbaH cala xEr
• waking up on good
• Romanization of Arabic from CallHome Egypt

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Constructions

• Identifying speaker intention rather than literal
  meaning for formulaic and task-oriented
  sentences.
• How about suggestion
• Why dont you suggestion
• Could you tell me request info.
• I was wondering request info.

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MT Handling of Constructions and Idioms

• Direct MT Approaches
• No Explicit treatment Phrasal Lexicons and
  sentence level matches or templates
• Syntactic Transfer
• No effective treatment
• Highly Lexicalized Structural Transfer rules
  can handle some constructions
• Trigger rule by matching against entire
  construction, including structure on SL side
• Rule specifies how to generate the correct
  construction on the TL side
• Semantic Transfer
• Analysis must capture non-compositional
  representation of the idiom or construction ?
  specialized rules
• TL Generation maps construction semantic
  structures to correct TL syntax and lexical words

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Take Home Messages

• Remember these types of language divergences as
  you learn about and apply the various steps in
  the MT system pipelines of different approaches!
• Ask yourself how capable these various steps and
  approaches are in addressing these types of
  divergences!
• Can the step/approach handle these divergences?
• If so, does it model the divergence at the
  appropriate level of abstraction?
• Keep these language divergences in mind when you
  analyze the errors of the MT system that you have
  put together and trained!
• Are the errors attributable to a particular
  divergence?
• What would be required for the system to address
  this type of error?

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Summary

• Main challenges for current state-of-the-art MT
  approaches - Coverage and Accuracy
• Acquiring broad-coverage high-accuracy
  translation lexicons (for words and phrases)
• learning syntactic mappings between languages
  from parallel word-aligned data
• overcoming syntax-to-semantics differences and
  dealing with constructions
• Effective Target Language Modeling

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Homework Assignment 1
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Questions