时态、人称、句式比较：

I am at home now.
Are you at home now?
I was at home this time yesterday.
Were you at home this time yesterday?

I'm doing my homework now.
Are you doing your homework now?
I was doing my homework at 5:00 p.m. yesterday.
Were you doing my homework at 5:00 p.m. yesterday?

He is swimming.
Was she in school last week?
They are riding bikes.
Were they on the mountain last month?