When is better than if for suggesting something will happen. If is better when suggesting something won’t happen.

This can be valuable when (not if) you’re writing insurance copy, crowd-sourcing or fund-raising advertisements, or any short or long-form content suggesting the possibility of a catastrophe, disaster, or dire consequences.
‘When you have a claim…’ is superior to ‘If you have a claim’ when writing what a policyholder might expect from you. But if you’re writing advertising for cars, you don’t want to suggest the possibility of a catastrophe, disaster or dire consequences. So you’d write ‘If you ever break a windscreen…’

Another word to be on the lookout for in the when/if mix is should. Adding a conditionality weakens the message. For example, ‘Should your policy be cancelled’ suggests the likelihood of cancellation whereas ‘If your policy is ever cancelled suggests it isn’t likely to happen.

Good copywriters avoid the if-should combination. Why? Adding ‘should’, introduces a dimension of uncertainty. It shifts the interpretation from the message maker to the message receiver.
Clever copy makes the target react positively or negatively. There should be no shades of grey.

As copywriters it needs to be our hands on the rhetorical advertising steering wheel, and they shouldn’t be sweaty with uncertainty.