Ответ:
1. The students were in the Russian Museum.
Were the students were in the Russian Museum?
The students weren't in the Russian Museum.
2. I am still working at my report.
Am I still working at my report?
I am not still working at my report.
3. I have known him for three years.
Have I known him for three years?
I haven't known him for three years.
4. Last summer we lived in the country.
Did we live in the country last summer?
We didn't live in the country last summer.
5. He writes letters every day.
Does he write letters every day?
He doesn't write letters every day.