They understand

OUT!!!

and

NO!!!

The first causes the instant reaction of scuttling feet and a fight to see who can get out of the door first. The second causes the tail and head to go down. I've tried saying other words in the same tone of voice and they don't react, except to look confused, so I'm sure they understand the words themselves.

They also understand (I think)

"I'm sorry, really, really sorry, forgive me."

When I accidentally bump one of them in my face with my elbow, or step on a foot while trying to clamber over another cat. They stop moving away, watch my face and listen, and then move forward for the comforting fuss.

Most not only know their names but their pet names. Dagda,for instance, knows that when I say "where's my big guy" that I'm looking to pay attention to him.

Like Barbara's cats, mine also know when I'm thinking "vet" or "medication", even though I keep my face bland and my demeanor calm.