You (save) money on your electricity bill if you didn't leave the lights on.
You would save money on your electricity bill if you didn't leave the lights on.
You (not fail) the exam if you'd bothered to study.
You wouldn't have failed the exam if you'd bothered to study.
If I (not miss) the train, I wouldn't have been late for my job interview.
If I hadn't missed the train, I wouldn't have been late for my job interview.
If we (speak) to the head teacher, do you think it would make a difference?
If we spoke to the head teacher, do you think it would make a difference?