Cambridge Grammar of English Hardback with cd-rom a comprehensive Guide

If in ellipted and non-finite clauses

If

also appears in ellipted and non-finite clauses in more formal contexts:

If wet, the match will be played on the indoor courts. 

(ellipted conditional clause)

If paying by direct debit, please make sure that you notify your bank at least one

month in advance. 

(non-finite conditional clause)

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If and when

If

and when are not the same. In unreal conditionals, only if, not when, may be

used. When is used to refer to something the speaker knows will happen at

some point in time:

If we win the lottery, we’ll give up our jobs and fly to the Caribbean.

(speaker does not know the event will happen)

When Georgina comes in, tell her I want to speak to her.

(speaker knows Georgina is coming)