1. In a grammar checking system, a determiner checking system for detecting improper referents of a noun phrase in an input sentence regardless of whether the sentence as a whole can be parsed, comprising: means for determining the parts of speech of the words in said sentence to provide parts of speech tags including a trigram part of speech tagger having means for ascertaining the probability of the part of speech of a candidate word; means, responsive to said means for determining the parts of speech of said sentence for identifying noun phrases, said noun phrases having constituents making up said noun phrase, said noun phrase identifying means including means for traversing said input sentence backwards, identifying via part of speech tags the immediately preceding noun, said immediately preceding noun being a head noun and as such being the right most word of the immediately preceding noun phrase, and continuing to work backwards to identify the rest of the words in said immediately preceding noun phrase, thus to identify the left boundary of said immediately preceding noun phrase so as to identify the start of a noun phrase and the end thereof, thereby to permit determination of number, whether the noun is a mass noun, is a singular non-proper noun, is part of an idiom or is a proper noun; and, means responsive to the identification of a noun phrase for detecting missing determiners, extraneous determiners, or lack of agreement in number for the constituents of said noun phrase.