I know whose
I know who you're talking about.
Encontré una nota en mi escritorio, pero no sé de quién es.
I found a note on my desk, but I don't know who it's from.
Sé de quién es esta bicicleta.
I know whose bike this is.
I know who you’re talking about.
Encontré una nota en la mesa, pero no sé de quién es.
I found a note on the table, but I don't know who it belongs to.
No sé de quién estás hablando.
I don't know who you're talking about.
I know whose car this is.