Respuestas
1. She made a cake -> She didn't make a cake
2. The girls sang beautiful songs -> The girls didn't sing beautiful songs
3. The boys swam in the sea -> The boys didn't swim in the sea
4. I told you a story -> I didn't tell you a story
5. Mary saw you at a café -> Mary didn't see you at a café
6. He wore a blue shirt on Monday -> He didn't wear a blue shirt on Monday
7. Susan taught English -> Susan didn't teach English
8. Tim read books after school -> Tim didn't read books after school
9. The postman rang the bell -> The postman didn't ring the bell
10. We spent 10 days in Italy -> We didn't spend 10 days in Italy
Por si acaso, cuando te piden que pases una oración a "simple past tense" lo unico que tienes que hacer es pasar los verbos de la oración a pasado, puedes buscar la forma pasada de un verbo en google si no la sabes.
Y para pasar una oración a negativo solo tienes que añadir didn't antes del verbo, y si en la oración está el verbo "was" (era / estaba) lo tienes que pasar a "wasn't" (no era / no estaba). No soy muy buena explicando pero bueno xD
Y ya, buena suerte y no olvides practicar eso!