"If: Most dictionaries' usage notes for if are long and involved; it might be English's hardest conjunction. From experience born of personal humiliation, I inform you that there are two main ways to mess up with if and make your writing look weak. The first is to use if for whether. They are not synonyms—if is used to express a conditional, whether to introduce alternative possibilities. True, abstract grammatical distinctions are hard to keep straight in the heat of composition, but in this case there's a wonderfully simple test you can use: If you can coherently insert an "[or not]" after either the conjunction or the clause it introduces, you need whether. Examples: "He didn't know whether [or not] it would rain"; "She asked me straight out whether I was a fetishist [or not]"; "We told him to call if [or not? no] he needed a ride [or not? no]." "
Excerpt From: David Foster Wallace. "Both Flesh and Not: Essays." iBooks.
