Maggot said:
Cats can't really undrestand language. She can tell what you're saying from your tone of voice - not the actual words.
They learn to associate sounds with actions/outcomes though, if words and phrases are used consistently and result in the same outcome every time, they will learn what is going to result after the phrase is spoken. When there is a reward or consequence involved, they will learn very quickly.
Radar understands the following (and this is not just fanciful imagining on my part, some of it is deliberate training which IS possible with cats if you are consistent):
No
Let go (he will retract his claws)
No scratchy-scratchy (stops him scratching on the chair)
Treat? (he will jump onto the pedal bin to wait for a cat treat)*
On the bin (as above)*
Do you want cuddles? (if he does, he will run and jump onto my lap)
Where's Mr Mousy? (he will bring a toy mouse so we can play fetch)
Do you want dindins? (he will run to the kitchen to wait for me to open the cat food)
Where does it go? (said when dinner is served - he will run to the spot where his foodbowls are placed, prevents him from tripping me en route)
Both of mine also know their own names and will come when called. Sonic is a bit younger and doesn't understand as much yet.
* I have trained him to jump onto the pedal bin for treats, very useful in getting him out from behind the fridge when necessary as that boy thinks with his stomach.