Can someone explain the difference between these two words? I was told by one person that cải tiến describes an improvement to something that is already good and makes it even better, and that cải thiện describes an improvement to something that is really bad and is just bringing up to normal level. But it seems maybe there are other differences too. For example, one person told me that cải tiến should only be used with physical objects like machines, or something with engines, but then I found some examples where this is not true. And I still don't know all the limitations or restrictions of cải thiện.Can anyone help explain?