Is the personal pronoun actually required for a grammatically correct sentence? Can I just say: "ні, не знаю," or does the я have to be there?
Yes, you can just say "Ні, не знаю", this is more spoken and lazy way. I would say it like this in 95% of times definitely.
Hey what do you know, I'm a contributor now :) Ні, не знаю is accepted.
It said ви was a typo and ти should have been used. Why?