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"When" is for certain future events; "if" is for uncertain or hypothetical ones.
Side-by-side comparison
"When" is used when you are certain something will happen; it is just a matter of when. It expresses certainty about the future event.
Call me when you arrive. I will be waiting.
When I retire, I want to travel the world.
She cried when she heard the good news.
"If" is used when the occurrence is uncertain or conditional; it may or may not happen. It expresses doubt or dependence on a condition.
Call me if you need anything. I am here.
If it rains tomorrow, we will cancel the picnic.
What would you do if you won the lottery?
Common mistake
"I will call you if you arrive" implies doubt that the person will arrive at all; "I will call you when you arrive" assumes they will.
Trick to remember it
- •Key question: is it certain to happen? Yes = "when" (certainty, matter of time); possible but uncertain = "if" (doubt).
- •"When I graduate" = I will graduate (just a matter of time); "If I graduate" = I'm not sure I will.
Quick rule
- •When = certain event (matter of time).
- •If = uncertain event (matter of possibility).
Did you know?
After "when" and "if" talking about the future, English uses the present simple, not "will": "When I arrive, I will call you", never "When I will arrive". The future is already marked in the main clause, so the time clause stays in the present. The same rule applies after "as soon as", "before", "after" and "until".
Practise what you learned
Exercise 1 · Pick the right one
_____ I get home, I'll call you. I should be back around seven.
Exercise 2 · Pick the right one
_____ it rains tomorrow, we'll cancel the barbecue.
Exercise 3 · Form the word
Related word pairs
"By" indicates a deadline (no later than); "until" indicates continuous duration up to a point.
"Since" marks the specific point when something started; "for" indicates how long something has lasted.
"Unless" means "if not" or "except if"; "if" is the neutral condition that can be positive or negative.
'A' is used before consonant sounds; 'an' is used before vowel sounds.
Bryan López
Founder of SpeakUP Academy