when and if

A person who says when (referring to the future) is sure that something will happen. A person who says if is unsure whether it will happen. Compare:

• I’ll see you at Christmas when we’re all at Sophie’s place. (We are certain to be at Sophie’s place.)
• I’ll see you in August if I come to New York. (Perhaps I’ll come to New York, perhaps not.)

To talk about repeated, predictable situations and events (in the sense of

‘whenever’), both when and if can be used with little difference of meaning.

• When/If you heat ice it turns to water.
• When/If I’m in Liverpool I usually stay with my sister.