Using modal verbs in the past can be tricky since they can be used in different ways with slight but important differences in meaning.
The first way in which we use modals in the past is relatively straightforward and requires only a simple word change:
I can speak French (Present ability)
I could speak French (Past ability)
I have to go to Porto (Present obligation)
I had to go to Porto (Past obligation)
Some past modals can be formed with have and the past participle. When we use past modals using this structure, special meaning needs to be considered:
I should go to the party (Feel obliged)
I should have gone to the party (I didn’t go. Now I regret it)
He might help her (Future possibility)
He might have helped her (Modal of deduction)
Further information can be found on the EC Learn English Website (www.ecenglish.com). Have a look and then try to use past modals in your next composition. The more you use them, the more natural they will feel. Good luck!