Formal vs. Informal
Here's a note that may be unfamiliar to most Duolingo users, it's about the three forms of "you." Hindi/Urdu has three forms, and in most cases, you'd not want to use the wrong type. Using both the scripts + Romanizations here:
तू/تو/tuu - least formal. Generally, I'd say avoid using this (unless of course you're so well acquainted that you may even use mild abuses playfully). Also never use it with elders, because it's just plain rude. Grammatically, you'd use this with third person forms (tuu kartaa hai for instance).
तुम/تم/tum - informal, but more formal than 'tuu' above. Use for friends and those of your age group. Mere acquaintances may or may not take it as impolite. Grammatically, it is used with second person forms (tum karte ho)
आप/آپ/aap - most formal. The standard address to use for elders and seniors in rank. Also the safest option to use because though it may sound awkward to a native speaker if you address a very close friend like this, it does not carry the risk of being misinterpreted as impolite or rude. Grammatically, it is used with the third person plural/honourific forms (aap karte hain) because technically, it is honourific speech.
Tum/aap can be understood as the analogues of to/shoma in Farsi/Persian or antum/ant(a/i) in Arabic or du/sie in German or tu/vous in French.
A cultural insight (perhaps?) could be the fact that I use only the last two with everyone I know. I cannot recall ever addressing anyone as 'tuu.'
I too have a propensity of calling others as tum or aap. I have never EVER addressed someone as tuu anytime before. Also, the others, who have a habit of calling others "tuu", address me as "tum" or "aap" and never "tuu". That's the same way I talk to them- "tum/aap". It shows reverence.
"TUU" IS RUDE!