1.They intend to make this beach a clean and beautiful place again

-> They plan to make this beach a clean and beautiful place again

2.The students started to collect old clothes for their charity programme last week.

-> The students have collected old clothes for their charity programme last week.

3.We make the air dirty because we use the car all the time

-> If we use the car all the time, we will make the air dirty.

4.If I don't leave now,I'll miss my train

-> Uncless I leave now, i'll miss the train.

5.I won't be able to do anything unless I have a quiet room

-> If I don't have a quiet room, I won't be able do anything.

8.Don't forget to turn the tap off when you are brushing your teeth.

III

1.They plan to make this beach a clean and beautiful place again.

2.The student have started to collect old clothes for thier charity programme since last week.

3.If we use the car all the time, the air will be dirty.

4.Unless I leave now, I won't miss my train.

5.If I don't have a quiet room, I won't be able to do anything.