Modal auxiliary verbs are followed by the base form of the main verb even in the past tense.
Even in the past tense, modal auxiliary verbs are followed by the base form of the main verb.
Let's see some examples:
- She could swim when she was five.
- They might have finished the project yesterday.
- He should have called me earlier.
- We would have gone to the party if we had been invited.
- You might have forgotten your umbrella.