Am I right?
1.The past of 'can' is 'could'
2. The past of 'must' is 'had to' or 'must' but with the meaning of 'conclusion' it is 'must have + p.p.'
3. The past form of 'shall' is 'should.'
4. The past of 'had better' is 'had better.'
5. The past of 'may' is 'might.'