Can we use "not" but the meaning isn't "not"?

Question

I'm quoting from a novel Harry Potter and the Prisoner of Azkaban.

'Ask McGonagall if you can go this time, Harry, the next one might not be for ages -'

I understand that in that context, "ages" means a very long time, so I'm sure I can substitue the sentence like this:

The next one might not be for a long time.

But, why it sounds like a negation? I have another copy of the novel in my native language, and the translation translated it without a negation. That means, Harry's opportunity to go might come for a long time. So, can you tell me why no is there?

Answer 1

"Be" in this context means "occur" or "happen" or "come about." "For" has the meaning of "until." You are correct that "ages" means "a long time," but in this context it really means "a long time from now." So:

The next [opportunity to go] might not occur until [a long time from now]

Something "occurs" at a single point in time; you cannot say it "occurs for a long time [from now]." You have to say it will occur a long time from now. But the "might" means it is uncertain whether it will occur a long time from now or not, and in order to convey that uncertainty you have to use "might not... until" as single construction:

The next opportunity will happen a long time from now.
The next opportunity might not happen until a long time from now.

That is just how the construction works.

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Your translation "come for a long time" does not make sense in this context, because "come" means "occur" or "happen" and that is, again, an instantaneous thing. There is another meaning of "come" that is sexual, and in that context "come for a long time" could make sense, but you don't mean it that way!