Similar to several Asian languages and cultures, there is a formal way of addressing elders versus the informal way of addressing people who are as old or younger than oneself.
In order to address someone older than me, I'd have to use the 'respectful' personal pronoun aap, whereas for someone my age or younger, I'd use the pronoun tum, and for someone who is even more known to me, the informal pronoun tu (similar to tu found in French). Quite unfortunately, these pronouns are also used to assert one's superiority over the other from both a financial and marital perspective. In India, one tends to address people who work blue collar jobs with the informal tum and tu. Husbands generally address their wives with the informal pronoun, whereas wives address their husbands with the formal aap. I should mention that I do not subscribe to these views and the same is true of a lot of people of my generation. While the use of formality to assert superiority is less common in marriages and relationships today, it is fairly common in elder generations (my grandparents address each other in this way).
Interestingly, Sanskrit, the language that Hindi is derived from, does not have different forms of personal pronouns based on the age of the person being addressed or respect one is trying to convey. The same is true of many other Indian languages, such as Bengali (the language of West Bengal, which is the state I am from).
It is deemed disrespectful to address a professor, parent, grandparent and even an elder sibling with the informal greeting. It is therefore a requirement of the language to use the formal pronouns when speaking or writing.
Replies