Why does this second conditional statement use the passive voice when the clauses are swapped?

Question

I came across this in a text book yesterday and it's been on my mind since. To be fair, it's not the best sentence anyway, but it did make me think.

"If a tornado hit Rex's house, it would destroy it."

If you swap the clauses - The house would be destroyed, if a tornado hit it.

Why does the main clause turn to passive voice when its position is swapped? I've been racking my brains, but I don't recall seeing this before with conditional statements.

The only reason I can think of is that the double use of IT in the original main clause doesn't work when you swap it round. Thus for it to make sense you need to talk about something directly.

Thanks

Answer 1

If you simply swap the clauses, you get:

• It would destroy it, if a tornado hit Rex's house.

But of course we usually use pronouns such as it after the referent noun is stated (retrospective anaphora), not before (anticipatory anaphora) as in the sentence above. Revising the sentence to follow more common practice results in :

• A tornado would destroy Rex's house, if it hit it.

The if it hit it clause is not particularly felicitous, but no passive conversion is involved. As @Kate Bunting points out, the passive is used only if you make the house the subject of the main clause instead of the tornado.

Answer 2

In your first sentence, the subject is a tornado - what it would do if it hit the house.

In the second version, the subject is the house - what would happen to it if it were to be hit by a tornado.