They got the children back alive only because they paid the ransom at once.
If they hadn't paid the ransom at once, they wouldn't have got the children back alive.
The examiner read the passage very quickly, so the candidates didn't understand it.
If the examiner hadn't read the passage very quickly, the candidates would have understood it.
They weren't wearing life-jackets; perhaps that's why they were drowned.
If they had been wearing life-jackets, they wouldn't have been drowned.
He didn't get to the top of his profession, perhaps because his wife didn't encourage him.
If his wife had encouraged him, he might have got to the top of his profession.
My house is guarded by two Alsatian dogs. That's the only reason it isn't broken into every night.
Unless my house were guarded by two Alsatian dogs, it would be broken into every night.
The flats are not clearly numbered, so it is very difficult to find anyone.
If the flats were clearly numbered, it would be easier to find anyone.
You don't wipe your feet, so you make muddy marks all over the floor.
If you wiped your feet, you wouldn't make muddy marks all over the floor.
I live near my office, so I don't spend much time travelling to work.
If I didn't live near my office, I would spend much time travelling to work.
The exit doors were blocked so people couldn't escape from the burning hall.
If the exit doors hadn't been blocked, people would have been able to escape from the burning hall.
The astronauts didn't walk very far on the moon because they were hampered by the thick dust.
If the astronauts hadn't been hampered by the thick dust, they would have walked further on the moon.
You should take more exercise because that's the way to get fit.
If you took more exercise, you would get fit.
The accident happened because the driver in front of stopped so suddenly.
If the driver in front hadn't stopped so suddenly, the accident wouldn't have happened.
I haven't got a key, so I can't get in.
If I had a key, I could get in.
Throw a stone into water and it sinks.
If you throw a stone into water, it sinks.
I didn't have any breakfast, and now I'm hungry.
If I had had breakfast, I wouldn't be hungry now.
Road travel is cheaper than rail travel in this country, so we have lots of traffic jams.
If road travel weren't cheaper than rail travel in this country, we wouldn't have so many traffic jams.