On whether you should write whether
One of my big nits as an editor is the slurring of fhe fields of meaning for words. So it irks me that "therefore" and "thus" are used interchangeably because they can convey different meanings, which makes them more useful tools. (The difference, tout court, is that "therefore" means "because of this" and "thus" means "in this way".) Don't get me started on "hence"!
Generally, I automatically "correct" these words to the usage I prefer as a small rebellion against killing their usefulness. I use the scare quotes because it's not wrong as such to write "I don't like him, thus I punched him" (but it's ugly!) but I greatly prefer "therefore" in that sentence.
Two words whose meaning has slurred together are "if" and "whether". This is particularly common in Australia, and I find myself "correcting" "if" to "whether" often, at least daily, sometimes several times in a day. Here, the justification is clearer than it is for "thus" to "therefore" because in formal writing, "if" is unacceptable where "whether" should be used. (The spoken language is somewhat different, and I use the idiom I grew up with, which favours "if".)
There are three cases for if/whether, simply stated:
A/ Sentences that need "if", where "whether" would simply be incorrect.
If I see him, I will tell him.
If the siren blows, you can leave work.
Here, "if" introduces a condition that does not imply a choice. Often it could be rendered "in the event that" (but please don't render it that way).
B/ Sentences that need "whether", where "if" would make the sentence ambiguous.
I don't know whether he's coming tomorrow or Wednesday.
means that he is coming on one of tomorrow or Wednesday but you don't know which.
Were you to write:
I don't know if he's coming tomorrow or Wednesday.
the sentence can then mean that he may be coming tomorrow or Wednesday or at some other time, and you don't know which.
"Whether" always implies at least two cases. It often introduces one case, and leaves the other implied.
I don't know whether he's coming (or is not coming).
Tell me whether you like my shoes (or you don't like my shoes).
This leaves us with a simple rule for deciding whether to use "whether". If you could append "or" plus the opposite of the case you are giving, use "whether".
It's then clear that case A sentences must use "if" because they only offer one condition, not two or more cases. See the difference between:
If the train comes, leave town.
Whether the train comes or not, leave town.
C/ Sentences in which 'whether" is more correct but people use "if".
I don't know whether he's coming on Friday.
I don't know if he's coming on Friday.
Check if you have any messages.
Check whether you have any messages.
We can tell we need "whether" in this last sentence because there is an implied clause "or you do not have messages".
Should you write "whether or not"? Generally, the rule of English applies that one should not use redundant words. In the same way that one writes "to" for "in order to" or "period" for "period of time", you ought not to use "whether or not" where "whether" alone would suffice.
One uses "whether or not" when we mean to say that both conditions under consideration apply. For instance, when M says he prefers blondes, I might say "I like women whether or not they are blonde", or you might ask whether I am keeping my child at home tomorrow if she is coughing, and I answer "I'm keeping her home as a precaution whether or not she's coughing". In this sense, it is clearly the same as "regardless whether".