How to use whether and if properly

A basic guideline is to use IF when you have a conditional sentence and WHETHER when you are showing that two alternatives are possible. Some examples will make this more clear.

For example, “The meteorologist didn’t know WHETHER the snowstorm would arrive on Monday or Tuesday.” Because I used “whether,” you know that there are two possibilities: snow will fall on Monday OR snow will fall on Tuesday.

Now see how the sentence has a different meaning when I use IF instead of whether:

“The meteorologist didn’t know IF the snowstorm would arrive on Monday or Tuesday.” Now, in addition to snowing on Monday or Tuesday, it’s possible that the snowstorm would not hit the region at all.

Here’s another example:
“Call the school principal if the snowstorm will arrive on Monday.”
“Call the school principal whether or not the snowstorm will arrive on Monday.”

The first sentence is conditional: call the school principal only if snow is going to fall. The second sentence is not conditional: call the school principal either way.

However, there are some circumstances when WHETHER and IF can be used interchangeably; this is possible when answering a yes/no question. For example:

“Are you going to the movies?” (This can be answered with a YES or NO.)
— I am unsure whether I will be going to the movies.
— I am unsure if I will be going to the movies.
(In both sentences, the meaning is that I may or may not go to the movies.)

Did you turn off the oven when you were finished baking cookies? (This can be answered with a YES or NO.)
— Eve wondered whether she had turned off the oven.
— Eve wondered if she had turned off the oven.
(In both sentences, the meaning is that EVE may or may not have turned off the oven.)

When do you add “or not,” as in “whether or not”? Adding the “or not” helps to show that both options are equally possible.

BASIC RULE
Use WHETHER when you have two distinct choices .
Use IF for conditional sentences.