Unfortunately, in Hindi you have to memorize gender of the nouns. ALthough ending -a is usually masculine, while -i is feminine, there are many, many exceptions.
OK in english you is for everyone like for elders or for younger
But in Hindi AAP is for elder than you
TUM is for same age of you and
TU is for younger than you
Not exactly its not defined you have to usee that way only. 'Tu'is not considered very respectful word so if you are meeting a stranger smaller than you, you shouldn't say tu/tum. also tu and tum is same thing just depends on the sentence
I wrote "आप कितना पानी पिते हैं?" Duo didn't mark it wrong; but it corrected my पिते as पीती। Why shouldn't it be, when आप is gender neutral and पिते is also gender neutral, treated as correct? Or is it that we must always treat आप or तुम as "Female" gender?
Your answer is correct, Duolingo doesn't have enough correct answers.
आप is gender neutral
पीते is masculine
पीती is feminine
Since gender is not known both answers should be accepted