We are familiar with the concept of "do support", where the verb do is used as a modal verb. It can be found frequently in Shakespeare and before and it is claimed to derive from the Celtic languages of Britain as referenced here. It is suggested that a different verb with the same meaning was used in the Brythonic (Welsh-like) languages, most commonly in declarative lists such as "jumping, shouting and screaming did he".
But neither this Brythonic use nor the Shakespearean use match the most significant feature of do support in Modern English, namely that we have to use it in most questions and negatives.
So where does the rule saying that we have to use it in questions and negatives come from? I can't find anything anywhere.