1.We should try to recycle rubbish.

2.Don't throw plastic bags because you can reuse them.

3.You should use reusable bags when you go shopping.

4.If the river is polluted, the fish will die.

5.If people cycle more, there will be less pollution.

We should try to recycle rubbish

-> should + V nguyên mẫu : nên làm gì

Don't throw plastic bags because you can't reuse them.

-> thì HTĐ , diễn tả sự thật 

You should use reusable bags when you go shopping

-> should+ V nguyên mẫu : nên làm gì

If the river is polluted the fish would die 

-> câu ĐK loại 2 

if people cycle more, there will be less pollution.

-> câu ĐK loại 1