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