John swam in the last summer отрицательная форма

John didnt 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. Thats why we use didnt.

Other example: Jane didnt go to Paris last year.

                       Tom didnt pass his examination two weeks ago.

                        Ann didnt enjoy her holidays last month.

                        Students didnt have classes last week.