1. Your mother and father are on vacation and you want to know when they are going to return.
2. You arrive at your friend's house to celebrate her birthday party. She meets you at the door.
3. The class is without a teacher and the children are noisy. The principal enters the classroom.
4. You introduce your friend Carlota to your mother.
5. At at train station in Spain, to buy a return a ticket from Barcelona to Madrid. The ticket wishes to know if you want a first class or second class.
In each situation, you must respond accordingly. That is why you have to respond formally or informally when necessary.
The difference between "tú" and "usted" depends on the formality. "Tú" is used when talking to friends or relatives (informal) while "usted" is used when talking to older people or a person of high social rank (formal).