>>8,19
No, really, grammar checkers are universally pieces of crap, no matter to what extent they try to interpret and parse English. The problem is that the language, as all natural languages, is highly ambiguous and rule sets defined in a grammar checker will probably never be able to take into account all of the intricacies of English.
I'm saying this as an English major, proofreader, and former TA. Grammar checkers give people a false sense of security that their text is correct, and that is not the case in far too many situations to make using them worthwhile. LEARN the language; don't rely on machines to do your learning for you. Turn that useless thing off.
Take a look at this:
http://papyr.com/hypertextbooks/grammar/gramchek.htm
Even a dedicated grammar checking program will only find roughly half of the most common grammatical mistakes, and the substandard and shoddy checkers in modern word processing applications are probably doing more harm than good by suggesting spurious changes to already-correct grammar.
See also:
http://seattlepi.nwsource.com/business/217802_grammar28.asp
http://itre.cis.upenn.edu/~myl/languagelog/archives/005061.html
http://itre.cis.upenn.edu/~myl/languagelog/archives/005404.html
http://writinghood.com/style/grammar/the-decline-of-english-grammar/