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.