"I do not know the woman."
Translation:Ik ken de vrouw niet.
Well, I tried to figure it out myself and found a rule in a book (the book is in Russian so this is my translation) that says that "geen" is used to deny a noun and it replaces indefinite article "een". In that sentence the woman has definite article so that might be the reason. But may be not :) I hope someone with better knowledge of Dutch will come to the thread and answer.