1. Use fresh economically, or there will be a shortage of water in a few decades.

=> If you don't Use fresh economically ,there will be a shortage of water in a few decades.

2. The bus was late, so Lan was late for school

=> Lan was late for school beacause the bus was late.

3. "Let's keep the air unpolluted", the Town President said.

=> The Town President suggested that. we should keep the air unpolluted.

4. I like painting.It is in this room

=> I like the painting of this room.