Is it okay to say "When was the last time you slipped?" ?
Or do you have to add "... on something", thus "When was the last time you slipped on something?" ?
You don't have to add "on something" for that. Imagine you are in a hospital and fell on the ice, so the doctor says, 'When was the last time you slipped?'
She doesn't have to add "on the ice."
