1) I've got to get up early.
Why have you got to get up early?
I've got to get up early because my friend is calling me. I must answer her (him).
2) I have to stay at home today.
Why do you have to stay home today?
I have to stay at home today because my mum will come soon and we will go for a walk.
4) I've got to buy new shoes.
Why have you got to buy new shoes?
I've got to buy new shoes because my old are not good. My Dad told me to buy new.