Grammar Reference

When and if

The main difference is to do with certainty. Compare these:

1) If the coin lands on tails, I'll eat these sweets.

2) When the coin stops, I'll know what to do.

In the first sentence, we don't know what will happen. The coin could land on heads or tails. Both consequences are possible, so we use if.

But in the second sentence, we are certain that the coin will stop. It's a question of when it stops, not if it stops.