In English, the word "if" prototypically marks conditionals. However, it can also mark indirect questions as well as lack of knowledge about polar statements:
Do you know if he will arrive within the hour?
I don't know if he can make it.
This might be my non-nativeness that influences me, but it seems a positive statement about knowledge gets weird with 'if', and 'whether' would be preferrable:
?I do know if he can make it, but I won't tell you.
I do know whether he can make it, but I won't tell you.
Now, much as subordinating conjunctions like these offer up a rich vista of potential rules, what I really wanted to introduce here was a thing that certainly some language has done, and I bet it's not even all that unusual - but most readers are probably not aware of it.
Consider making a decision... whether to do something.
Decide if you want to sell it/whether you want to sell it.
This could quite naturally have its own conjunction - and naturally, these three could have some overlaps in certain syntactic and semantic contexts.
Things that easily could affect their distribution are: verbal mood (especially conditional and imperative), negative, and potentially the verb of the matrix clause.
Further, one could of course have some subtle morphological variation on them depending on whether there's negatives involved, or whether the matrix clause has an imperative, etc.
