Put the verbs into Past Simple. Use the modal verbs and their equivalents: 1. He must work hard to finish his experiment. 2. I can devote myself to scienti fic work. 3. You may repeat this experiment. 4. He must illustrate this law by several experiments. 5. She can study automation in the college. 6. They may carry out these experiments at the laboratory.​

Day after day, the Earth is more and more devastated, giving humanity its wealth. Every day millions of tons of oil, natural gas, coal, peat and other minerals are pumped out. A man just devours them ruthlessly, and yet they are not eternal! Every day, factories and factories are being built that poison the atmosphere, releasing millions of tons of pollutants into the air, without thinking at the same time that we are digging a hole for ourselves. Not only air gets polluted. How many rivers, lakes, reservoirs and seas became a victim of emissions of toxic waste. Take, for example, Lake Baikal. This is the most burning example of human negligence.

1. Boris wasn't writing a letter at 6 o'clock.  He was reading a book at 6 o'clock. 
2. It wasn't raining when we returned home. It was snowing. 
3. The wind wasn't blowing from the west. It was blowing from the east. 
4. It wasn't snowing the whole day yesterday. It was raining.
5. The boys weren't fighting when we came up to them. They're playing.
6. Ann wasn't waiting for Nick when we saw her. She was waiting for Max. 
7. The rain wasn't beating agains the window when I shut the door. It was beating agains the door when I shut the window. 
8.The farm wasn't running along the street when we saw a child. He was standing without moving. 
9. We weren't sitting on the grass when they came up to us. We were standing and looking for a dry place. 
10. The boys weren't playing football from 2 till 4 o'clock. They were at the swimming-pool.