"When" is for certain future events; "if" is for uncertain or hypothetical ones.
At the time that. The speaker assumes the event WILL happen.
Call me when you arrive. I will be waiting.
Llamame cuando llegues. Estare esperando.
When I retire, I want to travel the world.
Cuando me jubile, quiero viajar por el mundo.
She cried when she heard the good news.
Ella lloro cuando escucho la buena noticia.
In the event that. The speaker does not know whether the event will happen — it is conditional.
Call me if you need anything. I am here.
Llamame si necesitas algo. Estoy aqui.
If it rains tomorrow, we will cancel the picnic.
Si llueve manana, cancelaremos el picnic.
What would you do if you won the lottery?
¿Que harias si ganaras la loteria?
Common mistake
The classic trap: "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. Spanish "si" maps cleanly to "if" but Spanish "cuando" + subjunctive also maps to "when + future" in English, confusing the choice of word.
Trick to remember it
Key question: is it CERTAIN to happen? If yes, use "when". If it is POSSIBLE but uncertain, use "if". "When I graduate" (I will graduate, it is just a matter of time). "If I graduate" (I am not sure I will). This completely changes the speaker's level of confidence.
Quick rule
When = certain event (matter of time). If = uncertain event (matter of possibility).
Did you know?
In English, neither "when" nor "if" take a future tense in their clause: "When I WILL arrive" and "If it WILL rain" are wrong — correct is "when I arrive" and "if it rains" (present with future meaning). This sounds strange to Spanish speakers used to "cuando yo llegue" (future subjunctive), but in English the simple present does the same job.
Exercise 1 · Pick the right one
I will send you the file _____ I get back to my computer later.
Exercise 2 · Pick the right one
_____ you see him at the party, tell him I said hello.
Exercise 3 · Form the word
When = certain event (matter of time). If = uncertain event (matter of possibility).
The classic trap: "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. Spanish "si" maps cleanly to "if" but Spanish "cuando" + subjunctive also maps to "when + future" in English, confusing the choice of word.
Key question: is it CERTAIN to happen? If yes, use "when". If it is POSSIBLE but uncertain, use "if". "When I graduate" (I will graduate, it is just a matter of time). "If I graduate" (I am not sure I will). This completely changes the speaker's level of confidence.
Bryan López
Founder of SpeakUP Academy