Ответ:

1. They must have left to Moscow for a conference.

2. They might have left to Moscow for a conference.

3. They may have left to Moscow for a conference.

4. They couldn't have left to Moscow for a conference.

5. They could have left to Moscow for a conference.

6. They needn't have left to Moscow for a conference.

7. They had to leave to Moscow for a conference.

8. They will leave to Moscow for a conference.