conditionals: what's the difference between these two sentences?
Solution 1:
1) I think that both cases are talking about a fire that never took place. They essentially mean the same thing. If there had been a fire then it would read "When one night their house caught fire..." or just "One night their house caught fire ..." without the conditional "if"
2) Whilst the first two versions describe a situation that may no longer exist, the sentence in the present tense implies that it is ongoing.