1) Is someone waiting for you?
Someone isn't waiting for you.
2) Am I going to invite somebody for dinner?
I amn't going to invite somebody for dinner.
3) Is he somewhere on the seaside?
He isn't somewhere on the seaside.
4) Is somebody coming to your house?
Somebody isn't coming to your house.
5) Have I found somebody's pen?
I haven't found somebody's pen.