1. There was a loud noise next to my door last night,
so I couldn't sleep.
=>I couldn't sleep  last night because there was a loud noise next to my door
2. They have learnt English for four years.
=>English has been learned for four years 
3.The tsunami happened. Many people in this air became homeless.
The tsunami made many people in this air became homeless
4.This factory doesn't treat its waste well, so there is a  lot of water pollution.
If this factory didn't treat its waste well, so there wouldn't a  lot of water pollution.
5. Because it rained heavily, the road in front of my house was flooded.
The road in front of my house was flooded due to the heavy rain