Modals in the past: had to, didn't have to, couldn't, was/were able to

We use had to to express an obligation in the past.

• I had to make a report. (I was obliged to do this)

We use didn't have to to express lack of obligation in the past.

• He didn’t have to come at 9. (He was not obliged to do this)

We use could to express an ability or permission in the past.

• He could go out after 9 pm. (He was allowed to do this)

We use couldn't to express lack of ability or permission in the past.

• I couldn't swim when I was 6. (I wasn’t able to swim)

We use was/were able to to express an achievement in the past.

• She was able to get this job. (She managed to get this job)