1. My brother didn't ride a bike last summer. - My brother rode a bike last summer.

2. You didn't swim in the Atlantic ocean last year. - You swam in the Atlantic ocean last year.

3. Children didn't wear school uniform two month ago. - Children wore school uniform two month ago.

4. Pushkin didn't write poems. - Pushkin wrote poems.

Здесь не сложно, просто нужно знать вторую форму глаголов и уметь применять её