1.He lost his umbrella yesterday.

→ He didn't lost his umbrella yesterday.

2.I am doing my homework now.

→ I am not doing my homework now. 

3.They left the door open last night.

→ They didn't leave the door open last night.

4.We were eating dinner at this time yesterday.

→We were not eating dinner at this time yesterday.

5.She will buy a new english dictionary the next day.

→ She will not buy a new english dictionary the next day.

6.The children went to school by bus.

→ The children didn't go to school by bus.

7.Ann's brother is going history at university.

→ Ann's brother is not going history at university.

8.Mr.Nam will visit his grandparents tomorrow.

→ Mr.Nam will not visit his grandparents tomorrow.

9.They usually play football at this stadium.

→ They don't usually play football at this stadium.

10.My father was working in a bank.

→ My father wasn't working in a bank.

11.Tom's mother often helps him with the homework.

→ Tom's mother doesn't often help him with the homework.

12.Angela did her homework very quickly yesterday.

→ Angela didn't do her homework very quickly yesterday.

- Answered by Meett1605