"Je ne sais pas de quoi tu parles" (or 'de quoi vous parlez' to be more polite)
Sérieux (if what you are talking about is masculine) or serieuse (if what you are talking about is feminine)
Tu es incompétent if you are talking to a man Tu es incompétente if you are talking to a woman
If you're talking about an object that is masculine (in case you don't know, objects are either feminine or masculine in french) you'll say "Celui-là" if it's feminine "celle-là"
"We know" in French is "nous savons".
Vous savez(connaissez)
I know what I am talking about.
Sérieux (if what you are talking about is masculine) or serieuse (if what you are talking about is feminine)
Presuming you're talking to a girl you know well, it's TU ES BELLE.
you just say i dont know what you're talking about
Je m'ennuie de vous parler is i miss talking to you in french!
Venez manger.(If you are talking to only one person, and you know them very well the you could say: viens manger.)
Ciel
hoiunnyt becoduu
Vous ne savez pas ce que vous parlez.
You don't say, enjoy "it". They know what you're talking about. You say: Amusez-vous bien! (Polite form) Amuse-toi bien! (Familiar form)
i dont know it
Tu es incompétent if you are talking to a man Tu es incompétente if you are talking to a woman