I've known Mr Smith for many years.
I have known her for two years.
You're not even phased, you must have known all along!
The doctor is known to everyone in the village.
The cause of the fire is not known.
She is known as the Japanese Picasso.
If I had known about your illness, I could have visited you in the hospital.
Had I known about it, I would have told you.
Had they known what was about to happen, they would have changed their plans.
How long have you known her?