As per my understanding with grammar, 1 is correct:

  1. I will not stop until it is finished.

  2. I will not stop until it is not finished.

  3. I will stop until it is finished

However, for 2, I always contend against accepting that the first not is negating the second not.
Am I correct that 2 can be written as 3? Please correct my grammar.

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  • The issue is not so much about grammar as with the meaning of the word until -- "until it is not" does not make a semantic element -- "so long as it is not" does. – Kris Jan 31 '15 at 13:24
  1. I will not stop until it is not finished.

However, I always have difficulties accepting that the first "Not" is negating second "Not" and the second line can be written as:

  1. I will stop until it is finished.

I don't think that this is a question of grammar, but rather one of logic. And I must disagree that 2 means the same thing as 3; they mean two entirely different things.
(2 means absolutely nothing at all for practical purposes.)

2. If you deconstruct 2, then there is clearly something that is in the process of being done or constructed, specifically whatever "it" is. This may or may not be being done by the speaker; that isn't clear. ("I will not stop" does not automatically imply that whatever the speaker is doing is contributing to the completion of "it", though it would commonly be taken that way.)

Here's the problem. If "it" is not finished, then the speaker has cause to stop. Right now. Immediately. Because the stated condition has already been met; "it" is already "not finished".

Once "it" is finished it can never be "not finished" again; time's arrow runs in only one direction only. (See any book by Einstein or Brian Cox.) So if "it" has already been finished, the speaker cannot stop, ever, simply because the condition of "it" not being finished will not exist again in the future. ("It" can be destroyed, but that is not the same thing as "not finished".)

For this reason 2 is, if not actually meaningless, certainly descriptively useless.

3. Deconstructing 3 yields a quite different result. In this case whatever the speaker is doing cannot be finishing "it", since if the speaker stops doing it (and is the only one doing it) then "it" will never be finished. This implies that one or more other parties are finishing "it". It also implies that the speaker will cease whatever he or she is doing until those parties finish "it".

To summarise then:

  • 2 means: if it is not finished, I will stop now. Nothing is said about when (or whether) I will start again. On the other hand if it is already finished I will never stop.

  • 3 means: I will stop now, and remain stopped until such time as someone else finishes it, then recommence whatever I am doing.

  • Thank you @Alan K. So overall the below sentence is right: I will not stop until it is finished. – codebased Jan 31 '15 at 3:51
  • 1
    Perhaps I am stripping a chest of drawers of its finish in order to refinish it. I've almost got it completely finish free; just a few more spots to go. My wife asks if I want to stop for lunch. I say, "No, I will not stop until it is not finished." Ok, I probably wouldn't say that. I'd say "No, I can't stop until I'm finished and it isn't." – Jim Jan 31 '15 at 6:15
  • Not even of logic. – Kris Jan 31 '15 at 13:25

In classical logic, not not p is equivalent to p -- the negation of a negated sentence is equivalent to that sentence. Your example "I will not stop until it is not finished", however, is not of that form, so the two "not"s do not cancel. You might also want to take into account that "until" is equivalent to "while not".

  • It makes sense that until means a condition (second part of the sentence) has not matched yet and thus the action can happen (first part of the sentence). "I will not stop until it is finished" is the right sentence. – codebased Jan 31 '15 at 5:09
  • 1
    What's "the action"? The first part of that sentence is "I will not stop". Failing to stop is not an action. – Greg Lee Jan 31 '15 at 5:20

"until" works these two ways

1) An action is in process. It will continue until X happens, then it will stop. This is expressed as a positive:

"I will do [whatever] until [x]."

Or the equvalent negative: "I will not stop doing [whatever] until [x]."

2) An action has not started, but it will (or at least, can) start after X happens. This is expressed as a negative.

"Do not start your exam until I give the signal."

"I will not drive forward until the light turns green."

To express this as a positive, you would not use "until" (asymmetrical, but as Alan K pointed out, tiime goes in only one direction).

{When/After} I give the signal, you may start your exam.

{When/After} the light turns green, I will drive ahead."

//////////// As others have pointed out, your second sentence, with the double negative, makes no sense.

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