What is the difference between 'I really don't know.' and 'I don't really know.'?
I don't really know is used to say that you are not certain about something,
e.g I don't really know what he's up to. I haven't heard from him for ages.
I really don't know is used to say that you definitely do not know something, especially when someone has asked you about it, e.g I can't answer that, I really don't know.