4. The world's energy resources are limited so we should use them economically. 

-> so : nên, đi với mệnh đề
5. The country could face an energy crisis if demand continues to rise. 

-> câu điều kiện loại I diễn tả hành động có thật ở hiện tại hoặc tương lai
6. You can't borrow my dictionary unless you bring it back on Monday. 

-> unless = if...not

7. The cost of installation is very high, but solar domestic heating systems are economical to use. 

-> but : nhưng
8. Solar panels are placed on the roof of a house and the Sun's energy is used to heat water. 

-> and : và, là từ nối 2 vế tương ứng
9. I don't agree with a lot of his teaching methods; however, he is a good teacher. 

-> however : tuy nhiên