Ответ:
1) I played the piano yesterday.
I didn't play the piano yesterday.
2) Alex worked in the garden last week.
Alex didn't work in the garden last week.
3) They danced at school three days ago.
They didn't dance at school three days ago.
4) We cooked a cake yesterday.
We didn't cook a cake yesterday.
5) My sister visited me the day before yesterday.
My sister didn't visit me the day before yesterday.
6) Mike played soccer at school.
Mike didn't play soccer at school.
7) The children helped their mother.
The children didn't help their mother.