Student or Learner
I know we use the structure "ask FOR" when it's about a thing. But then, can we say: "Can I ask you something? " or do we have to say "Can I ask you for something?"?
Same, if we want to point out that we don't need somebody's help, can we say "I didn't ask you anything" or do we have to say "I didn't ask you FOR anything? "?
If it's information you want, then use "Can I ask you something?" - How do I get to London? What's the time? When did the Second World War end? Are you still dating that great guy?
However, I would say that I am unlikely to say "Can I ask you for something?" even if I wanted a physical thing. Personally I would say:
Can I ask you something?
Yes, of course.
Great. Can I borrow a ladder please?
As for the second half of the question, if you are rejecting someone's help, then you would be more likely to say "I didn't ask you for anything", or simply "I didn't ask for your help".