Questions & Answers about उस बच्ची के बाल लंबे हैं, लेकिन उसका चेहरा छोटा है।
Because उस is the oblique form of वह.
In Hindi, when a noun or pronoun comes before a postposition such as का / के / की, में, से, etc., it usually changes to its oblique form.
Here, बच्ची is followed by के in उस बच्ची के बाल, so वह becomes उस.
- direct form: वह बच्ची = that girl
- oblique form: उस बच्ची के... = of that girl / that girl’s ...
Here के shows possession, like ’s or of in English.
So:
- उस बच्ची के बाल = that girl’s hair
The basic possessive set is:
- का for masculine singular
- के for masculine plural
- की for feminine singular/plural
In this sentence, बाल is treated as masculine plural, so के is used.
These are two different but very normal ways to show possession in Hindi.