When do I use "If" and when do I use "Whether"?

"If" = conditional (if X happens, then Y). "Whether" = alternatives (yes or no, either way). Mandatory: "whether" + infinitive, preposition + "whether", "whether" as subject.

What is the most common mistake with "If" and "Whether"?

"She couldn't decide if to go" is wrong: before an infinitive only "whether" works: "She couldn't decide WHETHER TO go". Also after a preposition: "the question of WHETHER it is safe". And: "I'll come if you invite me" (only if) is very different from "I'll come whether you invite me or not" (regardless).

How do I remember the difference between "If" and "Whether"?

Picture doors: IF = one door that only opens IF the condition is met (one possibility). WHETHER = two doors both open (the answer could be yes OR no, both are on the table). One conditional path? IF. Two open alternatives (yes or no)? WHETHER. WHETHER required: before infinitives, after prepositions, and as a clause subject.

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IfvsWhether

'If' introduces conditions; 'whether' introduces alternatives or is used when 'or not' is implied.

Side-by-side comparison

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"If" introduces a condition: something happens only if something else is true. The second part depends directly on the first.

If the project is approved by Friday, we can start the development phase next week.
Let me know if you need any additional information before making your decision.
I was not sure if she had received my message, she had not replied after two days.
She asked if there were any seats left for the evening show at the theatre.
He wanted to know if she had already seen that film before they chose it for the evening.

Whether

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"Whether" introduces an alternative or an indirect question. It asks if something is true or not, or presents two options. It can be followed by "or not".

She could not decide whether to accept the job offer in London or stay with her current company.
The committee will discuss whether the new policy should apply retroactively or only going forward.
Whether or not you agree with the decision, it has already been implemented company-wide.
She could not decide whether to stay for the last session or catch an earlier train home.
The teacher asked whether we had all finished reading the chapter before the class.

Common mistake

•"She couldn't decide if to go" is wrong: before an infinitive only "whether" works: "She couldn't decide WHETHER TO go".
•Also after a preposition: "the question of WHETHER it is safe".
•And: "I'll come if you invite me" (only if) is very different from "I'll come whether you invite me or not" (regardless).

Trick to remember it

•Picture doors: IF = one door that only opens IF the condition is met (one possibility). WHETHER = two doors both open (the answer could be yes OR no, both are on the table).
•One conditional path? IF. Two open alternatives (yes or no)? WHETHER.
•WHETHER required: before infinitives, after prepositions, and as a clause subject.

Quick rule

•"If" = conditional (if X happens, then Y).
•"Whether" = alternatives (yes or no, either way).
•Mandatory: "whether" + infinitive, preposition + "whether", "whether" as subject.

Did you know?

In legal and diplomatic English, the if/whether distinction is critically important. "The agreement will take effect if both parties sign" (conditional: only if both parties sign) is radically different from "The agreement will take effect whether or not both parties sign" (it takes effect regardless). An if/whether error in a contract can have significant legal consequences because it completely changes whether a clause is conditional or unconditional. International contract lawyers consider this one of the most important distinctions in contractual English.