Here are the incorrect parts circled for each sentence:

**VIII. Mistake: Circle the incorrect part**
1. Are you keen **on**train your pets?
- **Incorrect:** B. on (should be "on training")
- **Incorrect:** C. train (should be "training")

2. The streets **in** the city aremuch crowded than those in the countryside.
- **Incorrect:** B. are much (should be "are more")

3. My sister is a teacher but she works in an international primary school.
- **Incorrect:** C. work (should be "works")

**IX. Verb forms:**
1. I always enjoy **________** to my grandfather. He always tells me great stories.
- **Incorrect:** A. to talk (correct answer is B. talking)

2. Jane prefers **________** music to listening to it.
- **Incorrect:** B. play (correct answer is A. playing)

3. I would love **________** to your party! Thank you for inviting me.
- **Incorrect:** A. come (correct answer is C. to come)