Source: Code: The Hidden Language of Computer Hardware and Software by Charles Petzold
Example:
Electrons from the chemicals in the batteries might not so freely mingle with the electrons in the copper wires if not for a simple fact: all electrons, wherever they're found, are identical. There's nothing that distinguishes a copper electron from any other electron.
That's a very interesting phrase. Could you please explain to me what it really means and how to use it?
The reason it seems odd is because a few words were left out, namely it were: what “if not for” really means is “if it were not for”.
So it is positing a hypothetical conjecture.
You may even see “if not for” written as “were it not for”. It again simply means “if it were not for”.
It is used to precede (come before) the reason the previous statement is not true or cannot be so. Think of it as "[This would be true/happen] if not for [this fact]."
Some simple (some admittedly contrived) examples:
"I would buy you lunch if not for the fact that I have no money."
"This glass would be perfect if not for the crack at the top."
"I would take a ride along the ocean road this afternoon if not for the fact that my motorcycle is out of fuel."
