How do you know when to use “if” or “whether”?

They are often interchangeable, but the meaning of the sentence changes slightly depending on which word you use. 

“If” is for when something is conditional. 

e.g. “Call me if you’re free after the Teams meeting.” >>> You only need to call me if the condition happens that you are free after the meeting – otherwise, you don’t need to call.

“Whether” is for when something has only two options, A or B, and the sentence needs to make sense if you add the “or not”. 

e.g. “I don’t know whether (or not) I will work late tonight.” >>> There are only two options – late or on time.

Another e.g. “Well, let me know whether (or not) you’re going to be free after the meeting.” >>> Again, there’s only two options – either you’re free afterwards, or you’re not free afterwards.

(C) TJ Withers-Ryan, 2022. Credit me when you share, thanks! 🙂