"We must open those windows with something."
Translation:Něčím ta okna otevřít musíme.
It is hard to explain. It sounds unnatural.
I added a slight variation "Musíme ta okna otevřít něčím." which sounds better to me, although the stress at "něčím" in the final position is unusual. Maybe someone will add more comments to your word order and perhaps we will add it later.