Originally Posted by HarryT
Word's grammar checker is a waste of space. Ignore it. (But not the spell checker!)
I would not go to that extreme... but you cannot really trust it. It will catch some things fairly well, others it will miss completely and some it will just plain misreport. If you have a decent grasp of grammar you can find it useful... but it can not even come close to replacing a 1/2 decent proof reader or copy-editor.
In short, hire an editor when your done before you publish regardless of what word says.