Here are the completed sentences based on the given simple sentences:

1. He is clever. He is active.
➔ He is clever and active.

2. I wash my face. I brush my teeth.
➔ I wash my face and brush my teeth.

3. You shouldn't eat much fast food. You shouldn't eat sweetened food.
➔ You shouldn’t eat much fast food or sweetened food.

4. Some students are jogging. Some students are talking.
➔ Some students are jogging while others are talking.

5. She wears a hat. She puts on some suncream.
➔ She wears a hat and puts on some suncream.

6. The Japanese study hard. The Japanese work hard.
➔ The Japanese study and work hard.

7. My cousin learns English. My cousin learns Japanese.
➔ My cousin learns English and Japanese.

8. The car is new. The car is comfortable.
➔ The car is new and comfortable.

9. Nam likes playing chess. His brother likes playing chess.
➔ Nam likes playing chess, and his brother likes playing chess too.

10. Eating lots of vegetables is good for your health. Eating lots of fruit is good for your health.
➔ Eating lots of vegetables and fruit is good for your health.

11. We have breakfast. We go to school.
➔ We have breakfast before we go to school.