This is serious but also somewhat tongue in cheek:

If Microsoft wants to supply Spelling, and particularly Grammar
checkers, why dont they learn what is correct and incorrect.
I do a lot of technical writing for our company, but am not a trained
writer.
Of all the words in the English language that I do not understand, it
is WHICH and THAT. I am never sure what to use, and when the
Spell/Grammar utility gets to one or the other, it detects an error,
and presents you with being able to choose either - they cant both be
correct. My proof reader was a college English major and she
constantly berates me for my misuse of these two words, amoung others.
Get with it Mr. Gates, with all the money Microsoft makes, the least
that you can do is hire someone who understands English grammar to fix
it.