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Wiktionary says

The sentences “I don't know whether or not I passed” or “I don't know whether/if I passed or not” are both correct.

The sentence […] but “I don't know if or not I passed” is incorrect.

Why is it so?

WHETHER OR NO: CONJ
3. used as a conjunction as a variant of whether
4. under any circumstances: he will be here tomorrow, whether or no.
Source: WordReference

OED: IF https://oed.com/oed2/00111435 , WHETHER https://oed.com/oed2/00284376

Mari-Lou A
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GJC
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2 Answers2

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"Whether or no" is an alternative to "whether or not" and it means exactly the same.

It appears in the nursery rhyme 'A frog he would a-wooing go':

A frog he would a-wooing go,
Hey ho! said Rowley,
A frog he would a-wooing go,
Whether his mother would let him or no.
With a rowley powley, gammon and spinach,
Hey ho! said Anthony Rowley.

Fowler (A Dictionary of Modern Usage, 1926) gives other examples of "or no" being used the same way:

Pleasant or no, it is true;

He must do it whether he will or no.

NGram shows "Whether or no" falling slowly into disuse over the last two hundred years. When it does appear nowadays it seems to be mostly in quotations from previous centuries.

Old Brixtonian
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  • Sum-of-Parts since NO (adverb) 4. not: whether or no. https://www.wordreference.com/definition/no – GJC Jun 26 '21 at 11:30
  • Furthermore, I don't know what meaning(s) of ''whether'' would whether or no be a variant of https://www.wordreference.com/definition/whether – GJC Jun 26 '21 at 13:30
  • @GJC: I don't know what you are saying here. What does "Sum-of-parts since" mean? "Whether or no" isn't a variant of 'whether'. – Old Brixtonian Jun 26 '21 at 22:38
  • https://imgur.com/a/bZEZ6ka. – GJC Jun 26 '21 at 23:25
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This is the rule according to Cambridge too:

To express an alternative, we can use or not with if and whether. With whether we can use or not immediately after whether or in end position. With if we use or not in end position only: e.g.
I called Bill to find out whether or not he really did go to Afghanistan.

I called Bill to find out whether he really did go to Afghanistan or not.

I called Bill to find out if he really did go to Afghanistan or not.

But people do use if not erroneously, sometimes just because they don't know the rule:

the phrase “whether or not”, as in “I was wondering whether or not you would come” is okay, but “if or not” in the same context seems not okay - google searches bring up 100 million hits for the first phrase, but just 15,000 for the second. This came up in a class I was in, and I was surprised because I do use “if or not” in informal speech (painintheenglish)

I am surprised even NGram finds instances of if or not. I'd recommend you avoid it though, it's not good English.

Here is what Macmillan says:

Use whether, but not if, immediately before ‘or not’:

  • The question is whether or not the government has the right to interfere.

So there you go. The answer is pretty much what @Hot Licks commented: "Because English!"

fev
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  • WhETHER OR NO: 1. used as a conjunction as a variant of whether 2. under any circumstances: he will be here tomorrow, whether or no https://www.wordreference.com/definition/whether – GJC Jun 25 '21 at 19:36
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    Yes, and? You kinda lost me there :-) – fev Jun 25 '21 at 19:39
  • Might whether or not be an idiom too? https://idioms.thefreedictionary.com/whether+or+not – GJC Jun 25 '21 at 19:43
  • The link you gave is for the case when "whether or not" is synonymous with "regardless"/"whatever happens". Your OP was about the case when "whether or not" can be interchangeable with "if ... or not" (considering one option of two or more). I am not sure your edit of the OP is helpful. What is the actual question now? – fev Jun 25 '21 at 19:47
  • But what meaning(s) of ''whether'' would whether or no be a variant of? https://en.wiktionary.org/wiki/whether#Usage_notes – GJC Jun 25 '21 at 19:49