I’d like a native speaker or a grammarian to answer my question on till/until. I cannot find an explanation to this topic. They say until is used in negative sentences and till is used in affirmative ones. But I can’t find an answer to this.
This doesn’t ring any bells with me. To pick two random examples, “I didn’t know until/till you told me” and “I’ll wait until/till tomorrow” both seem OK to me with either “till” or “until” (albeit I have a general preference for “until” in writing). Did you have any specific examples in mind? Perhaps there is some special case that I’m overlooking.
Hi dear Dozy 
I don’t have any examples. I’ve just heard the above-mentioned explanation, but have also seen many examples where both variants are possible, that’s why started to think over it. Thank you so much