6. The non-transitory computer-readable recording medium of claim 1 , wherein the detecting of the errors comprises when frequencies of appearance of the forward morpheme sequences are different from frequencies of appearance of the backward morpheme sequences, detecting the errors based on higher frequencies of appearance among frequencies of appearance of the forward morpheme sequences and the backward morpheme sequences.
7. The non-transitory computer-readable recording medium of claim 6 , wherein the detecting of the grammatical errors based on the higher frequencies of appearance comprises: calculating suitability scores using the higher frequencies of appearance; and detecting the grammatical errors by determining whether the suitability scores are smaller than a threshold or whether the suitability scores are smaller than the threshold by a predetermined value or more.