ABSTRACT
This chapter illustrates something of the difficulty of the Natural Language Processing (NLP) task by examining the range of different knowledge structures that are necessary for processing even the simplest of texts. Understanding a language means being able to match linguistic sounds to meanings. If English only consisted of a small number of sentences, say fifty, this would be easy. What is acquired is the language's grammar, the structural system which underlies each sentence. Grammatical knowledge is central to language processing. It is usual to identify five levels of grammatical structure. They are lexicon, Morphology, Phonology, Syntax and semantics. Sentences consist of words, strung together, one after another. For instance, in the sentence have phrasal structure, noun phrase and verb phrase. The phrasal structure of a language is described by phrase structure rules. Ambiguity of this kind is called structural ambiguity. There are various other sources of ambiguity; Lexical ambiguity and Scope ambiguities.
