didn't - сокращение от did not. После did/didn't глагол не меняется, т.е. остаётся в первой форме.
1. On Saturday I did play computer yames with my cousins
2. My mum didn't cook dinner last night
3. I did walk to school because there weren't any buses
4. They didn't dance at the party
5. My brother did travel to Ireland last summer