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Hi.

These are rules which [it] would be almost impossible to break.

I feel confident that it's optional, but if I were to include it, what would be the grammatical justification?

Thanks, - A.
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The grammatical structure is anticipatory it with important (and other similar adjectives) and an infinitive, but here it's in a subordinate clause.

To break these rules is impossible.
gives
It is impossible to break these rules.

The tense is different, of course, and almost is added.

It would be almost impossible to break these rules.

These are rules [it would be almost impossible to break them]

These are rules which it would be almost impossible to break ___.

CJ
Comments  
Thanks, CJ.