Whether introduces indirect questions and takes a to-infinitive only when the subjects match, as in I have not decided whether to resign, but requires a full clause when the subjects differ or a fact is being stated, as in I do not know whether he will come. It is never used with if plus infinitive.
Grammar Rule
Use whether + to-infinitive only when the subjects match; use whether + clause when subjects differ. Never if + infinitive.
Whether + Infinitive vs Whether + Clause: Rules and Examples
| Pattern | Requirement | Example |
|---|---|---|
| whether + to-infinitive | same subject for main verb and infinitive | I haven’t decided whether to resign. He asked whether to bring his own equipment. |
| whether + clause | different subjects, or stating a fact | I don’t know whether he will come. They debated whether the plan was feasible at all. |
Remember: use whether + to-infinitive only when subjects match (and never after if); use whether + clause when subjects differ or a full finite verb is needed.
Practice Test
Instructions
Answer each question in the test below choosing correctly between whether + infinitive and whether + clause. You will get a new random set of questions every time you retake the test, plus a downloadable results summary with your name and score at the end.
C1-57 "Whether" + Infinitive vs. "Whether" + Clause
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Frequently Asked Questions
When does whether take a to-infinitive?
Only when the subjects match, as in I haven’t decided whether to resign.
When does whether take a full clause?
When the subjects differ or a fact is being stated, as in I don’t know whether he will come.
Can if replace whether before an infinitive?
No, whether + to-infinitive is never replaced by if + infinitive.
What does whether introduce?
Indirect questions, as in they debated whether the plan was feasible at all.