What’s the difference between the words “if” and “whether”?

“If” is used for one scenario. “Whether” cannot be used in these sentences:-

  • If I lived on Mars I would be very lonely. ✔︎
  • You must stop if the light is red. ✔︎

“Whether” is used to indicate that there is more than one scenario:-

  • Whether I lived on Antarctica, the moon, or Mars, I would be very lonely. ✔︎
  • I don’t know whether I’ll take the train or not. ✔︎
  • You must stop whether the light is red or yellow. ✔︎

In these sentences “if” can also be used. Sometimes the meaning is the same. “If” can seem to imply that we are discussing one scenario with different variations, rather than different scenarios. This does not apply for the middle sentence:-

  • If I lived on Antarctica, the moon, or Mars, I would be very lonely. ✔︎
  • I don’t know if I’ll take the train or not. ✔︎
  • You must stop if the light is red or yellow. ✔︎

In the middle one above, we can miss out “or not”, keeping the same meaning. (We cannot do this with the other (conditional) sentences.):-

  • I don’t know if/whether I’ll take the train. ✔︎

You can’t say or write “if to” – you have to reword:-

  • I need to decide whether to go or not. (No “if” here.) ✔︎
  • I need to decide if I should go or not. (or “whether”) ✔︎

You can’t say or write “if or not” – the “or not” can go later in the sentence:-

  • I need to decide if I should go or not. ✔︎
  • I need to decide whether I should go or not. ✔︎
  • I need to decide if or not I should go. X
  • I need to decide whether or not I should go. ✔︎