For the sentence 'You could have fed the cat' I need to come up with a context from which to elicit the target language. The most simple idea I can think of is saying that my flat mate is very lazy and that one day I came home and the cat was very hungry - what do you think I said to my flat mate? - You could have fed the cat.
I'm having trouble with this though - wouldn't this be 'you SHOULD have fed the cat'??? Any help on this much appreciated!
"You knew I was going to be late home. You could have fed the cat!"
If you are really angry - "you could at least have fed the cat [even if you did nothing else about the place]"