If or when?

We use if to introduce a possible or unreal situation or condition. We use when to refer to the time of a future situation or condition that we are certain of:

• You can only go in if you’ve got your ticket.
• When I’m older, I’d love to be a dancer.

Compare

If Giles comes back to the office, can you tell him I’ve gone home.	The speaker does not know whether Giles is coming back to the office. It is possible, but not definite.
When Giles comes back to the office, can you tell him I’ve gone home.	The speaker is certain that Giles is coming back to the office.

To talk about situations and conditions that are repeated or predictable, we can use either if or when + present verb form:

• You can drive if you’re 17.
• If you don’t add enough wood, the fire goes out.
• When we go camping, we usually take two tents.
• She gets out of breath easily when she’s jogging.

Typical error

We don’t use when to introduce possible or unreal situations:

• Unfortunately, if you arrive too late, you are not allowed to take the exam because they don’t accept late enrolment.
• Not: … when you arrive too late …