"not until" means the time period "only after". "until" means the time period "up to the event or moment mentioned. "Not until I spoke to him did he look at me". We can reword this as: Only after I spoke to him did he look at me". Notice that "not until" is referring to the time period sometime AFTER I spoke to him. Now your 2nd sentence: "He should wait until he knows more about it." We can reword this sentence as: "He should wait until the moment/event that he knows more about it" Notice that "until" is referring to the time period BEFORE the event mentioned (the time he knows more about it).

Hi Katya, this is a bit confusing but I will try to explain. "Until" = a point in time, but it does not have to be specific. "Not until" = something will not happen, or did not happen, or cannot happen, until a certain specific point in time. Not until my 21st birthday will I learn how to drive a car. It was not until midnight that the train arrived. I do not have time to travel overseas now, not until my Christmas holiday break. The last sentence does not have a point in time (21st birthday; midnight; Christmas holiday) so it does not need the "not". I hope this helps and is not too confusing!

Just to let you know a bit more, the first two of the three sentences are written in a specific rhetorical expression called "inversion" or "anastrophe", which changes the order of words and phrase so as to emphasize some of them. You don't necessarily have to use it if it is too difficult for you.

You could say that, though you don't need to take the trouble to use a double-negative here. You usually just say "He had to wait until he knew more about it".

So, is the matter just in the past time? So, can I write: He didn't have to wait NOT until he knew more about it?