Look out for 2nd & 3rd conditional unreal sentences in the text. 
The end of the world could ve happened yesterday. What would you have done if you had known only a few hours before impact? Ok, so this is an exaggeration. The asteroid 2005 YU55 safely passed the Earth at 11:28pm on Tuesday. It is 400metres in diameter and last night it was closer to the Earth than the moon. Maybe it was because I had just watched the film Melancholia (about a planet colliding into Earth with only a few hours warning), but I kept looking out of the window until 11:30pm. If the world was going to end, I wanted to know about it! So the next day I wondered, If I had seen the asteroid coming to earth, what would I have done in my last few hours? I know, quite a morbid thought on a Wednesday morning. Anyway, to be honest, I m quite a laid-back person so I wouldn t have wanted to panic. I was also lucky because I was already with one person I loved, so I think I would ve called my family and just watched the world end from my balcony. What about the other teachers? Sarah, a relatively new teacher, was nice enough to respond to the question without thinking I was a weirdo: If there had only been a few hours notice, I would ve called all my friends to come over for one big end-of-the-world party! Roger, a distinctly older teacher replied: I was washing the dishes a few hours before the asteroid passed. If I d known it was going to hit Earth at this moment, I might ve finished the washing-up. It depended on if I had wanted to die knowing that I had finished my chores. Bren said about last night: If an asteroid was on target with the earth, I would make a bad joke like, "Well, don't worry about it. It's not the end of the world, is it?" And then I'd think about possibly asking for from the nearest babe. Maria, my current co-teacher, responded with: That was never going to happen. But if it had, I would ve taken the first flight to Africa to spend my last hours in the country I love the most Mali. Even if she d had 12 hours warning, she probably couldn t have taken a flight at such short notice anyway. Good thing that this never happened! Apparently the next time an asteroid gets this close to Earth will be in 2029 and it ll be the same one. So you have a long time to think, What would you do if it was going to hit the world?
2nd Conditional: If + past simple / past continuous, subject + would/wouldn t + infinitive
Use to talk about hypothetical situations in the present and future. You can use might or could instead of would for possibility
- If the world was going to end, I would want to know about it
- If an asteroid was on target with the earth, I would make a bad joke
3rd Conditional: If + past perfect, subject + would/wouldn t / could/could ve / might + have + past participle
We use to talk about hypothetical situations with a past consequence You can use might have or could have instead of would have for possibility
- If there had only been a few hours notice, I would ve called all my friends to come over for one big end-of-the-world party
- If I d known it was going to hit Earth at this moment, I might ve finished the washing-up
- If she d had 12 hours warning, she probably couldn t have taken a flight at such short notice anyway.