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Case roles describe the function of a noun in a sentence in relation to the main verb. Some languages use formal case systems, while others use syntax or word order. Linguists distinguish between surface and deep structure, which is important in natural language processing. The grammatical function of a noun can usually be determined by word order in English. However, the same case can have different semantic functions, making it difficult for computers to recognize. Natural language processing aims to program computers to detect the deep structure of human languages.
In linguistics, a case role describes the function of a noun in a sentence in relation to the main verb of the sentence. Some languages have formal case systems, where the noun’s form determines its case. The case systems of other languages are determined by syntax or word order. In both situations, linguists can make distinctions between a case’s surface structure and its deep structure, meaning that different nouns might appear to have the same case but actually serve different functions. The process of determining the deep structure of a case role is one of the goals of natural language processing.
In its most basic form, a case role is the grammatical function of a noun in a sentence, such as the subject noun, direct object, indirect object, etc. In English, you can usually determine these functions based on word order, since a sentence typically follows this pattern: noun, subject, verb, indirect object, direct object. Of course, there are many variations on this basic pattern, but the syntax of a sentence provides the information needed to determine the grammatical function of the noun. Inflected languages, such as Latin or Greek, use word endings to indicate grammatical function.
The situation is complicated, however, by the fact that nouns appearing in the same case may have different semantic functions. For example, the two sentences “The butler killed the cook with arsenic” and “The butler killed the cook with ease” each contain the preposition “with” followed by a noun. Logically, however, the two nouns do not have the same semantic function. “With arsenic” describes the tool with which the cook was killed, but “with ease” describes the way the cook was killed.
This example demonstrates the difference between surface structure and deep structure. The two sentences have the same syntactic construction and the objects of the two prepositions would probably be in the same formal case in an inflected language. This means that they have the same “surface structure”. On the other hand, they do not have the same “deep structure”, as the semantic meanings are different. The two nouns, therefore, do not have the same case role even though they appear to have the same grammatical role.
One application of the role of chance is in the field of natural language processing, which analyzes the relationship between human languages and computer languages. One goal of natural language processing is to program computers to detect the deep structure of human languages for information extraction, translation, or other purposes. While it is easy enough for a human to determine that “with arsenic” and “with ease” do not have the same semantic function, this difference is not easily recognized by a computer. For example, for a computer to translate the above sentence into a language where the two sentences would not have the same surface structure, an algorithm would be needed to determine the deep structure of the sentence.
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