This requires a lengthy explanation, so I have copied the following rules from: http://www.ehow.com/how_2122453_use-whether-if.html 1 Use whether when discussing two or more alternatives. For example, "I'm not sure whether I know how to use the word 'if' properly." In this sentence the alternative, "or not," is implied. It is appropriate to include the alternative in this form: "I'm not sure whether or not I know how to use the word 'if' properly." This usage can sound awkward so the implication is sufficient. 2 Reserve if for circumstances with only one condition. A mother might tell her son, "You can have dessert if you eat your vegetables." The son having dessert depends solely on his consumption of vegetables, and is therefore conditional. 3 Use whether after prepositions. "I am uncertain about whether we should go to the dinner party," is proper usage of whether following the preposition "about." "I am uncertain about if we should go to the dinner party," is incorrect usage. When unsure about whether to use whether or if, use both in a sentence and say it to yourself out loud. With the preceding sentences, the first sounds proper but the second sounds awkward. 4 Answer yes or no questions with whether or if. For example, Sally asks, "Can you go shopping with me tonight"? Proper responses would be: "I'm not sure if I can go shopping with you tonight," or "I'm not sure whether I can go shopping with you tonight." 5 Choose whether with infinitives that come after "to." For instance, "Jenny doesn't know whether to study for her test or go to a movie with friends." The infinitive is study and is preceded by to. To say, "Jenny doesn't know if to study for her test or go to a movie with friends," is grammatically incorrect and true to the test, sounds funny when spoken.

If means just condition, it is used more widely. Whether means that there is some alternative and usually accompanied by OR. Example: You can take an apple, if you want. You must eat this apple, whether you want it or not.

Use 'if' to introduce a condition. In all other circumstances, use 'whether'. A condition is something that must be satisfied before something else occurs. If it rains, bring in the tables. (If it rains = condition) Add a point if I get the right answer. (if I get the right answer = condition)

google it

Before an infinitive with TO use whether not if........I am not sure whether to buy.................. After a PREPOSITION use whether not if.........It depends on whether the letter...............