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I have been bothered by the question whether 'unless' and 'if not' can be used interchangeably. I think they can have the opposite meaning, but I am not sure. Could you support my opinion with some examples or theory?

I was also presented with a sentence:

Unless I had lost the umbrella, I would have go out now.

But I know unless is used to refer to the future, anyway, might it be correct?

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closed as not constructive by FumbleFingers, MετάEd, kiamlaluno, Robusto, David Schwartz Dec 12 '12 at 2:54

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That is one strange sentence. Usually "unless" is paired with a negative, as in "Unless you do as I say, you won't get to go to the circus." Even in cases where it is paired with a positive, the subordinate clause speaks of a deficit: "Unless I'm wrong, you're going to go to the circus." "Unless we fail in our task, we will all be rich." –  Robusto Dec 11 '12 at 20:33
    
RE: That is one strange sentence - I wonder if maybe the O.P. is trying to say, "Unless I've lost my umbrella, I'm going out [into the rain] now." That might make more sense. –  J.R. Dec 11 '12 at 21:34
    
Yes, it does, but if it were the sentence you mention I wouldn't have asked the question :) –  anifanti Dec 11 '12 at 21:42
    
Who presented that sentence to you, then? –  J.R. Dec 11 '12 at 21:49
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If and unless have opposite meanings, but if not and unless have identical meanings. –  tchrist Dec 11 '12 at 22:53

3 Answers 3

You yourself have presented an example where they cannot be used interchangeably.

The sentence:

If I had not lost the umbrella, I would have gone out,

is good English. The sentence:

*Unless I had lost the umbrella, I would have gone out,

sounds very strange to a native English speaker. The reason is that you can only use "unless" in the past if the verb is habitual. For example:

The pizza never arrived hot unless I tipped the delivery boy,

sounds fine.

Put those two sentences into the future, and they both sound fine:

If I do not lose the umbrella, I will go out,
Unless I lose the umbrella, I will go out.

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Thanks for spelling this out like that; it was a real eye-opener. Your starred sentence can only be grammatical if it is habitual, and I have no idea why. The OED sheds no light on the matter, defining unless as except or if..not. It does include some older citations that are hard to make sense of from a contemporary point of view, such as this one: 1710 Swift Jrnl. to Stella 8 Oct., ― I was at a loss to-day for a dinner, unless I would have gone a great way. –  tchrist Dec 12 '12 at 13:20

Got your point. That said, a minor tweek to the conditional statements will make the phrases interchangeable:

Examples:

If your answer is not received by 8 tonight, we will not go to school tomorrow.

Unless your answer is received by 8 tonight, we will not go to school tomorrow.

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you can edit you original answer to include this extra detail. –  Matt Эллен Dec 11 '12 at 21:36
    
And can you back up Robusto or can it be always used not matter if the sentence is positive or negative or a person wants to use unless referring to the past? –  anifanti Dec 11 '12 at 21:39

"Unless" and "if not" are not interchangeable. They are two different beasts but I will try to clarify their usage and leave the final answer to you!!!

Unless starts a conditional statement.

Example: you refuse to do something unless something else happens.

"If not" is used to interject or clarify something or when the possibility of a better or different situation is possible.

Examples: Investing in the business will yield 10% more, if not, you get your money back. Let's meet today if not, tomorrow!

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You need to be careful with punctuation here. "Let's meet in Rome: if not, in Paris" means "Let's try to meet in Rome, and if that doesn't work let's meet in Paris"; "Let's meet in Rome, if not in Paris" means "Let's try to meet in Paris, and if that doesn't work let's meet in Rome." –  DJClayworth Dec 11 '12 at 20:50
    
Thanks but I meant 'If not' as the one used in the conditionals, as Unless it rains, we will go and if it doesn't rain we will go. I wrote 'if not' because of different auxuliary verbs we can have in conditionals. I do know what you are saying, but can you relate? –  anifanti Dec 11 '12 at 21:11

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