John swam in the last summer отрицательная форма

John didn't swim last summer.

In the negative sentences and past tenses we use the auxiliary did and plus the negative particle not in order to negate the sentence. That's why we use didn't.

Other example: Jane didn't go to Paris last year.

Tom didn't pass his examination two weeks ago.

Ann didn't enjoy her holidays last month.

Students didn't have classes last week.