1. An apparatus for automatically evaluating sentences, the apparatus comprising: an input sentence divider configured to divide a composed input sentence into morphemes which are the smallest units of a sentence; a part-of-speech tagging section configured to tag parts of speech to the divided morphemes; a syntax analyzer configured to analyze a syntax structure of the sentence based on the parts of speech and to output a syntax tree as an analysis result; a sentence evaluating section configured to evaluate whether the sentence is grammatically correct by using the syntax tree output by the syntax analyzer; and an error detector configured to generate a part-of-speech string by using an n-gram with respect to each part of speech tagged by the part-of-speech tagging section, to analyze the generated part-of-speech string based on a regular grammar rule and a context free grammar rule which are defined in accordance with connection relationship between the parts of speech adjacent to each other to detect an error of syntax analysis, and to provide a corrected draft for the detected error.