15. Underline the correct verb form in each of the following sentences.
1. People have always polluted / had always polluted their surroundings.
2. Before we came, they had discussed / were discussing the problems that threaten human life on Earth.
3. With the development of industrial cities, which put huge amounts of pollutants into small areas, the problem has become / had become urgent.
4. Our forests are disappearing / had disappeared because they are cut down or burnt.
5. She was relieved to hear that he had saved / saved those animals.
6. If nothing is done / had done about it, one day nothing will be able to live in the seas.
7. He said that he has developed / had developed new methods.
16. Fill in the table with the forms of the following verbs:
to roll, to remove, to break, to heat, to twist, to bend
Example: to draw
| Indefinite | Continuous | Perfect | Perfect Continuous | |
| Present | draw | are drawing | have drawn | have been drawing |
| Past | drew | were drawing | had drawn | had been drawing |
| Future | will draw | will be drawing | will have drawn | will have been drawing |