what's the difference between "shouldn't do" and "shouldn't be doing"?
All I can think of is that "shouldn't be doing" is used when you say to someone who is doing it right now. Is it right?

Question

Answer

100% correct!

Examples:

I am drinking too much wine *right now*. I shouldn't be doing that.

I eat sweets every day.  I shouldn't do that.

Answer

I shouldn't do that... can refer to what is happening now. Example: a child is doing something wrong or dangerous and mother says, "I shouldn't do that!"  It's a warning to stop what the child is doing at that moment. The full sentence would be, "I shouldn't do that if I were you!"