If introduces a condition. Whether introduces two possibilities, often with or not sitting in the background. If you finish early, check your work. She asked whether Science was optional. Diction pairs like this are in word choice.
Condition or choice
A real if-then: If the timer hits zero, the section ends. Two options, yes or no, this or that: whether. After verbs like ask, decide, know, and wonder, the ACT almost always wants whether, not if. I do not know if I should guess is the usual miss. Change it to whether.
Either and or mark a different two-way choice, in either versus or. Neither and nor are the negative pair, in neither nor. Grammar overall is in ACT grammar rules.
The bottom line
Real condition: if. Two possibilities, especially after ask or decide: whether.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.