1. An apparatus for detecting grammatical errors, the apparatus comprising: a sentence analyzer configured to break up a sentence into units of morphemes, tag the morphemes with parts of speech, and analyze a syntactic structure of the sentence based on the tagged parts of speech; a first error detector configured to generate part-of-speech sequences using n-grams of the tagged parts of speech, and detect first grammatical errors by analyzing the generated part-of-speech sequences based on grammatical rules; a second error detector configured to generate morpheme sequences by binding the morphemes in a preset n-window size, wherein n is an integer, and detect second grammatical errors according to frequencies of appearance of morpheme sequences identical to the generated morpheme sequences by searching examples from an example-based index database (DB); and an integrated error detector configured to determine final grammatical errors in the sentence by combining the detected first grammatical errors and the detected second grammatical errors, wherein the second error detector comprises a second error detection unit, the second error detection unit is configured to: search whether or not the morpheme sequences identical to the generated morpheme sequences are in the example-based index DB; extract the frequencies of appearance of the morpheme sequences identical to the generated morpheme sequences, and calculating respective suitability scores of the morpheme sequences based on the extracted frequencies of appearance; and detect second grammatical errors by determining whether the suitability scores are smaller than a threshold or whether the suitability scores are changed larger than a predetermined value, and wherein when the second error unit calculates the suitability scores, the second error detection unit is further configured to, when the morpheme sequences generated in the second error detecting step include both forward morpheme sequences and backward morpheme sequences, extract frequencies at which each morpheme of the morphemes is arranged with the forward morpheme sequences and the backward morpheme sequences, calculate suitability scores of each morpheme of the morphemes based on the extracted frequencies, and calculate an integrated suitability score in consideration of the suitability scores of each morpheme of the morphemes.