Quick Answer: Whether introduces alternatives or a reported yes/no question: I don’t know whether to go or stay. If introduces conditions or reported yes/no questions in informal contexts: If you come, I’ll be happy. In many sentences both work, but whether is safer in formal writing and with infinitives, prepositions, and at the start of a sentence. For a fuller definition, see the Cambridge Dictionary entry for ‘if’.

At a Glance: If vs Whether

WordPart of SpeechMain UseExample
ifconjunctionconditional; informal reported yes/no questionIf it rains, we stay. / I asked if she was ready.
whetherconjunctionalternatives (or not); formal reported yes/no questionI don't know whether to go. / Whether you like it or not.

Using If

If is a subordinating conjunction. Its primary use is to introduce a conditional clause — a clause that states a condition:

If you study hard, you will pass.

I'll call you if I'm late.

If she had known, she would have helped.

What would you do if you won the lottery?

If in Reported Questions

If is commonly used in indirect/reported questions, especially in informal speech:

She asked if I wanted more coffee.

I wonder if he remembers me.

Can you check if the door is locked?

Using Whether

Whether is used when two alternatives are being considered (often with “or”), when introducing indirect yes/no questions in formal writing, or when the clause is the subject or object of a sentence.

I don't know whether she will come or not.

Whether you agree or disagree, the decision is final.

The question is whether the plan will work.

She couldn't decide whether to accept the offer.

Whether + Infinitive

Whether is the only option (not if) when followed directly by a to-infinitive:

I didn't know whether to laugh or cry.

She couldn't decide whether to stay.

I didn't know if to laugh. (WRONG — if cannot precede a to-infinitive directly)

Whether After Prepositions

Whether must be used (not if) when the clause follows a preposition:

The argument was about whether the price was fair.

He wasn't sure about whether to apply.

The argument was about if the price was fair. (WRONG)

Common Mistakes

Mistake 1 — Using “if” with a to-infinitive

I didn't know if to go or stay.

I didn't know whether to go or stay.

Note: whether is required before to-infinitives. If cannot precede to + verb.

Mistake 2 — Using “if” after a preposition

The debate was about if the law should change.

The debate was about whether the law should change.

Note: after prepositions (about, on, of, regarding), use whether — not if.

Mistake 3 — Using “if” as the subject of a sentence

If he will accept is unclear.

Whether he will accept is unclear.

Note: whether (not if) is used when the clause functions as the subject of the main verb.

Related Confusing Word Pairs

Quick Quiz: If vs Whether

Choose the correct word for each sentence.

1. “___ you like it or not, the rules apply to everyone.”

2. “The question is ___ the plan will work.”

3. “I couldn't decide ___ to call or send an email.”

4. “___ it rains, we will cancel the picnic.”

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Frequently Asked Questions

What is the difference between if and whether?
If introduces conditions (“If it rains, we stay”) and informal reported questions (“She asked if I was ready”). Whether introduces alternatives (“whether to go or stay”), formal reported questions, and is required before to-infinitives and after prepositions.
Can if and whether be used interchangeably?
In reported yes/no questions, often yes: “She asked if/whether I was ready.” However, whether is required: (1) before to-infinitives: “I didn't know whether to go”; (2) after prepositions: “It's about whether...”; (3) as the subject of a sentence: “Whether he agrees is uncertain.”
When must I use whether and not if?
Use whether (not if): (1) before to + infinitive: “I didn't know whether to answer”; (2) after prepositions (about, on, regarding): “The question about whether...”; (3) when the clause is the subject: “Whether he is right remains to be seen”; (4) when emphasising alternatives with “or”: “whether to go or stay.”
What does “whether or not” mean?
Whether or not means “regardless of whether”: “I'll go whether or not it rains.” This phrase always uses whether, never if. You can also say “whether it rains or not.”
Can whether introduce a conditional?
No. Conditional clauses (if something happens, then...) use if, not whether. “Whether” only introduces alternatives or indirect questions, not conditions.
Is “I don't know if” correct?
Yes, in informal speech: “I don't know if he is coming.” In formal writing, whether is preferred: “I don't know whether he will attend.” Both are grammatically acceptable in most contexts.
What is an indirect question?
An indirect (reported) question is a question embedded inside a statement: “She asked whether I was ready.” The word order is statement order (subject + verb), not question order. Both if and whether introduce indirect yes/no questions; whether is more formal.
Can whether start a sentence?
Yes: “Whether you agree or disagree, the decision stands.” If is also possible at the start of a sentence, but only for conditionals: “If you agree, let me know.” Using if at the start of a subject clause (“If she will come is unclear”) is non-standard; use whether.
What is the difference between “if” and “in case”?
If introduces a condition that must be met. In case introduces a precaution: “Take an umbrella in case it rains” (= to be prepared, not as a condition). “Take an umbrella if it rains” means only take it if it actually rains.
How do if and whether appear in IELTS writing?
In IELTS Task 2, both appear frequently. Conditional if is essential for hypothetical arguments: “If governments invested more...” Whether is used in academic discussion of alternatives: “The question of whether technology improves learning...” Using whether in formal clauses shows awareness of formal grammar register.