1. In a grammar checking system in which sentences are first tagged as to parts of speech, a system for correcting incorrect auxiliary verb sequences having at least two verbs, comprising: means responsive to said tags for detecting starting and ending points of said verb sequence; means for identifying the verbs in said sequence; a directed acyclic graph containing a set of auxiliary verb sequences; means coupled to said directed acyclic graph for determining that said identified verbs are incorrect in accordance with the entries in said directed acyclic graph; and, means for correcting said identified verbs determined to be incorrect.