Use ‘whether’ when there is an alternative involved . . .
- I can’t decide whether I should leave or stay.
- Can you tell me whether you intend to pay me? (The alternative is not to pay me.)
- Use a moisture analyser to visually indicate whether the dehydrator is functioning within its designed range.
In most cases, do not use ‘or not’ with ‘whether.’ It is redundant.
- Please tell me whether
or not you are staying for lunch.
or not the printer is working will determine how much work we get done.
Use ‘if’ to express a condition.
- If you aren’t going, I’ll just stay home.
- If thrust moves to the negative, slowly close the ball valve.
- Fuel gas pressure will eventually be lost if total gas flow is isolated through the JT valve.