Sure! Here are the sentences changed into negative forms, questions, and their answers:

1. **Original**: He was planting trees in the garden at 4 pm yesterday.
- **Negative**: He was not planting trees in the garden at 4 pm yesterday.
- **Question**: Was he planting trees in the garden at 4 pm yesterday?
- **Answer**: Yes, he was planting trees in the garden at 4 pm yesterday.

2. **Original**: They were working when she came yesterday.
- **Negative**: They were not working when she came yesterday.
- **Question**: Were they working when she came yesterday?
- **Answer**: No, they were not working when she came yesterday.

3. **Original**: She was painting a picture while her mother was making a cake.
- **Negative**: She was not painting a picture while her mother was making a cake.
- **Question**: Was she painting a picture while her mother was making a cake?
- **Answer**: Yes, she was painting a picture while her mother was making a cake.

4. **Original**: Anne was riding her bike to school when Peter saw her yesterday.
- **Negative**: Anne was not riding her bike to school when Peter saw her yesterday.
- **Question**: Was Anne riding her bike to school when Peter saw her yesterday?
- **Answer**: Yes, Anne was riding her bike to school when Peter saw her yesterday.

5. **Original**: He was typing a letter when his boss went into the room.
- **Negative**: He was not typing a letter when his boss went into the room.
- **Question**: Was he typing a letter when his boss went into the room?
- **Answer**: Yes, he was typing a letter when his boss went into the room.

Feel free to ask if you have more questions!