"unless" and "if and only if not"

[YAMATO2201]

In my high school days, all my English teachers claimed that "unless" and "if not" were always interchangeable. But, some years later, I encountered a sentence to which such claim cannot be applicable. I thought about "unless" for some time and came to the conclusion that "unless" means "if and only if not". In order to confirm my conclusion, I examined some sample sentences from my textbooks.

Please look at the sentences below:

(A) I'll go unless it rains.
(B) I'll go if and only if it does not rain.
(C) I'll go if it doesn't rain.
(D) I'll be surprised if Mary doesn't win.
(E) I'll be surprised unless Mary wins.

My observations on these sentences are:

[1] (A) and (B) have the same meaning (according to my conclusion above), although I suppose that native English speakers would not use (B) under any circumstances.

[2] (C) is superficially different from (B), but I think that (C) connotes "I won't go if it rains." Hence (B) and (C) are the same in meaning.

[3] (D) and (E) are not interchangeable. I think that (E) is incorrect or absurd.

Am I right?

Thank you.

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This post has been corrected. The original post contains a careless mistake.
The second (D) in [3] in the original post should be (E).

 

[Tarheel]

You are mostly right. A,B and C mean pretty much the same thing. As for D, the speaker expects Mary to win and will be surprised if she doesn't. E makes no sense to me.

 

[GoesStation]

Conclusion 1 is more or less correct. If and only if is certainly unlikely, if not completely impossible.

Conclusion 2 is correct. The first part of conclusion 3 is correct, but sentence D is fine.

 

[YAMATO2201]

You are mostly right. A,B and C mean pretty much the same thing. As for D, the speaker expects Mary to win and will be surprised if she doesn't. E makes no sense to me.

As soon as I saw your reply, I noticed that I had made a careless mistake in my original post. The second (D) in [3] should be (E). I apologize.

Many thanks.

 

[YAMATO2201]

Conclusion 1 is more or less correct. If and only if is certainly unlikely, if not completely impossible.

Conclusion is correct. The first part of conclusion 3 is correct, but sentence D is fine.

I made a careless mistake in my original post. The second (D) in [3] should be (E). I apologize.

Thanks a lot.