"You drink water."
Translation:तुम पानी पीते हो।
My native language Dutch and also German have a somewhat similar system of honorifics. It is silly if you would only learn the top step of the system. Since the different honorific levels of pronouns come with different conjugations it is certainly useful to learn all of them. Think of reading novels or talking to children. I'm certain Indian children will feel odd when a foreigner adresses them with Aap.
I agree that people learning Hindi (or any language as a matter of fact) should learn all the various honorifics, it's an essential part of learning any language. What I don't agree on is the talking to children not using "Aap" part. Actually, from an Indian perspective, it's polite and not at all strange to use "aap" with kids. My mum uses "aap" while talking to the neighbourhood's kids and used to talk to me and my siblings when we were kids using "aap". With kids, I use "tum" or sometimes "aap" but every Indian is different. There's a billion of us, maybe I was raised uber-polite lol.
तु is the 2nd person most informal pronoun. Use it for kids, spouses(and I've also heard, used when addressing god). तुम is the medium level of informality Use it when address friends. Also the use of तुम vs. आप (the most formal second person pronoun) varies from city to city. Some use आप with the exact same people another city would use तुम.