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!
I also think it's also variable across regions. In some cities, and especially among the youth, it's pretty standard to almost never use "aap" except with elders.
In some regions, even elders (and family) members are referred to only using 'tum'! :D These things always make me wonder how diverse the Hindi belt really is in India.
India is diverse. Yes. There are more than 22 languages there like- Bengali, Gujrati, Tamil, etc. Hindi is their common language, just like English is the global language. Many family members call each other "tum" out of love.
You can never go wrong with aap, anywhere in India. But dont be surprised if strangers do spring an occassional tum on you
I usually use aap, even with people who are younger than me, because as someone who learned to speak Hindi when I was young, I am more used to it. However, tuu isn't completely rude and in some regions it's common so I wouldn't say to never use it- especially with friends.