What are the rules for changing modal verbs in reported speech?

The rules for changing modal verbs in reported speech are as follows:

1. When reporting statements or affirmative sentences, the modal verb is usually changed to the past tense. For example:
- Direct speech: "I can swim."
- Reported speech: He said he could swim.

2. When reporting negative sentences, the modal verb remains the same in the reported speech. For example:
- Direct speech: "I can't swim."
- Reported speech: He said he couldn't swim.

3. When reporting questions, the modal verb is usually changed to the past tense, and the word order is changed to a statement. For example:
- Direct speech: "Can you swim?"
- Reported speech: He asked if I could swim.

4. When reporting requests or commands, the modal verb is usually changed to the past tense, and the word order is changed to a statement. For example:
- Direct speech: "Could you help me?"
- Reported speech: He asked if I could help him.

5. When reporting offers or suggestions, the modal verb is usually changed to the past tense, and the word order is changed to a statement. For example:
- Direct speech: "Shall we go for a walk?"
- Reported speech: He suggested that we should go for a walk.

It is important to note that the reporting verb and the tense of the reporting verb may also affect the changes made to the modal verbs in reported speech.