Not if, but when…

The expression ‘not if, but when’ is a handy way to understand the difference between if and when when talking about the future. It signals that something – good or bad – is inevitable. In this expression, when, carries more weight to the speaker than the if.

You’ll often use this expression in situations where you’re confident an event will happen. For example,  instead of saying ‚if I get the job…‘ you’d say ‚when I get the job…‚ to express certainty. This phrase is frequently paired with expressions like  ‘it’s (not) a matter of’ or ‘it’s (not) a question of’ to emphasize inevitability.

Examples

• Not if I run a marathon, but when!
• It’s a matter of when I get the job, not if!
• It’s not a question of if, but when the next cyber attack will strike.

Grammar point – if and when for the future

If is used to express a possibility in the future, whereas when is used for things that will happen. 

Examples

• When I go out later, I’ll pick up some groceries. (certain to happen)
• If I go out later, I’ll pick up some groceries.  (might happen)

Form:

(if clause)	(main clause)
If + subject + simple present + rest of clause,	subject + will + bare infinitive + rest of clause.
If I go out later,	I’ll get some groceries.

If the main clause is first, there is no comma:

• I’ll pick up some groceries when I go out later.