1.  aren't going to play

2. isn't going to walk 

3. am not going to have

4. isn't going 

5. am going to watch

6. aren't going to go

7. am not going to finish

8. aren't going to buy

They (play) football, they're going to watch it.

1. • This sentence is already correct. It means they are going to watch a football game.

2. He (walk) to school, he's going to cycle.

   • This sentence is also correct. It means he is going to ride his bike to school.

3. I (have) lunch because I'm not hungry.

   • The correct form here is negative: I'm not going to have lunch because I'm not hungry.

4. She (go) to bed early because she's tired.

   • Correct: She's going to go to bed early because she's tired. (Or, more simply: She's going to bed early...)

5. I'm interested in athletics, so I (watch) the Olympic Games on TV.

   • Correct: I'm going to watch the Olympic Games on TV.

6. We (go) shopping, we're going to stay at home.

   • This sentence is a bit confusing. It seems contradictory. If they're going to stay at home, they're not going shopping. Perhaps you meant to say: We're not going to go shopping, we're going to stay at home.

7. I (finish) my homework tonight, I'm going to do it tomorrow.

   • This sentence is also contradictory. If you're going to do it tomorrow, you're not going to finish it tonight. Correct form: I'm not going to finish my homework tonight, I'm going to do it tomorrow.

8. I (write) a long email and then send it to my friend in Spain.

   • Correct: I'm going to write a long email and then send it to my friend in Spain.

9. We (buy) a new car because our old car is fine.

   • If your old car is fine, you probably won't buy a new one. Correct: We're not going to buy a new car because our old car is fine.