Training a dog not to bite when it's in pain requires a combination of patience, consistency, and positive reinforcement. Here are some steps you can take to train your dog:

1. Understand your dog's body language: Learn to recognize signs of pain in your dog, such as whimpering, growling, or showing aggression. This will help you identify when your dog is in discomfort.

2. Consult with a veterinarian: If your dog is frequently experiencing pain, it's essential to address the underlying health issue. Consult with a veterinarian to diagnose and treat any medical conditions your dog may have.

3. Do not provoke or handle your dog roughly: It's crucial to handle a dog in pain gently and avoid any actions that may increase its discomfort. Be careful not to touch or manipulate the painful area unless necessary.

4. Provide a comfortable environment: Create a comfortable and safe space for your dog to rest and recover. Make sure it has a soft bed or blanket, and minimize any potential stressors or triggers that could exacerbate its pain.

5. Use positive reinforcement training: Positive reinforcement training involves rewarding desired behaviors to encourage their repetition. When your dog is in pain and refrains from biting, immediately reward it with treats, praise, and affection. This will reinforce the idea that not biting leads to positive outcomes.

6. Teach the "leave it" and "drop it" commands: Train your dog to respond to the "leave it" and "drop it" commands. This will help redirect its focus from potentially biting or mouthing something when in pain.

7. Desensitize your dog to touch: Gradually desensitize your dog to touch around sensitive areas when it's not in pain. Start by gently touching these areas for short periods, rewarding your dog for calm behavior. Gradually increase the duration and intensity of the touch over time.

8. Implement a "no bite" command: Teach your dog a "no bite" or "gentle" command, associating it with positive reinforcement. When your dog displays signs of discomfort, use the command in a firm but calm tone to discourage biting behavior.

9. Seek professional help if necessary: If you're struggling to train your dog not to bite when in pain, consider seeking assistance from a professional dog trainer or behaviorist. They can provide tailored guidance and hands-on training techniques to address the specific issue.

Remember, training takes time, and each dog is unique. It's important to remain patient, consistent, and understanding throughout the process.