Do you often write letters?
Yes, I have been writing them since I was a child.
How often do you write them?
I usually write one each day.
Did you write one yesterday?
Yes, I have written one today, and I will write one again tomorrow.
When exactly was it written?
Were you writing it when your friend came?
Yes, I was writing it when he knocked on my door.
Are you still writing?
Yes, I'm writing it now.