Sure! Here are the completed sentences using the present continuous:

1. The boys are not doing their homework at the moment. They are watching TV instead.
2. Mai is reading about renewable energy, and Nam is doing a project on our local environment.
3. The music is getting louder and louder. Many people are putting their hands over their ears.
4. Her mother is not having a nap now. She is watering the plants.
5. My father is installing a biogas cooker in the kitchen, and the workers are putting solar panels on the roof of our house.
6. Lan always calls me when I am doing my homework.
7. The workers are putting the solar panels on the hill when it suddenly rains.
8. While we are looking for information about energy sources, the electricity is going out.
9. I often play the piano in the evening, but tonight I am watching TV.
10. Normally my father waters the vegetables in the garden, but today he is cooking in the kitchen.
11. They have a lot of work to do, so they are not going to town tonight.
12. He usually comes back home at one o'clock, but today he is working late.