1. I enjoy playing tennis, but my sister enjoys playing badminton. Corrected sentence: My sister enjoys playing badminton, but I enjoy playing tennis.

2. You may have an allergy, so be careful with what you eat and drink. Corrected sentence: You may have an allergy, so be careful with what you eat and drink.

3. It’s hot outside, but I decide to walk. Corrected sentence: It's hot outside, but I decide to walk.

4. Although I learnt hard, I didn’t get high grades. Corrected sentence: Although I studied hard, I didn't get high grades.

5. Because I did not study hard, I failed the final exam. Corrected sentence: Because I didn't study hard, I failed the final exam.

6. Although he behaved badly, he wasn’t punished. Corrected sentence: Although he behaved badly, he wasn't punished.

7. Because Sara has failed many times, she disappointed. Corrected sentence: Because Sara has failed many times, she was disappointed.

And here are the answers to the reorder exercises:

IX. 1. If he studies hard, he will get good grades.

Corrected sentence: If he studies hard, he will get good grades.

IX. 2. If they don’t leave now, they will be late for the concert.

Corrected sentence: If they don’t leave now, they will be late for the concert.