If that is true, then it should have accepted 'their' son as an answer - I understand that they are discouraging non-gendered words as they don't exist much in Hindi compared to English, and 'their' is technically plural, but in English it is used when the gender is uncertain
It seems that this course is introducing too many concepts too fast. I would appreciate more begining lessons with emphasis on reading the script, word order and gender. Those who are more advanced can test out to higher levels, and the beginers can have more practice before advancing.
The other languages i have studied on Duo start slow, with sentances of just a few words. Introduce only a few new words in each sentance and negatives are introduced in it's own section. Thank you for considering this.
"us" and "kā" are different parts of speech. It's "her + of". I think because uskā is us + kā, it helps to keep the separation. For what it's worth: 1) SOMETIMES (in non-standard orthography) we actually see "उस का" (i.e. with a space between). You can see evidence in this Google search: https://www.google.com/search?q=%22%E0%A4%89%E0%A4%B8+%E0%A4%95%E0%A4%BE%22&newwindow=1&bih=592&biw=1056&hl=en&sxsrf=ALeKk01nCHMF_nTBVwo-pH-Ab2rG1vhXjg%3A1619025327227&ei=r12AYNyjDY32-gT8yIrYDQ&oq=%22%E0%A4%89%E0%A4%B8+%E0%A4%95%E0%A4%BE%22&gs_lcp=Cgdnd3Mtd2l6EAMyBwguEBMQkwIyBAgAEBMyBAgAEBMyBAgAEBMyBAgAEBMyBAgAEBMyBAgAEBMyBAgAEBMyBAgAEBMyBAgAEBM6CQgAELADEAcQHjoHCAAQsAMQHjoICAAQBxAeEBM6BAguEBNQplJYtXZggnloAXAAeACAAYwBiAGHBJIBAzIuM5gBAKABAaoBB2d3cy13aXrIAQLAAQE&sclient=gws-wiz&ved=0ahUKEwict4us64_wAhUNu54KHXykAtsQ4dUDCA0&uact=5 2) In the orthography of Punjabi (which is a very very similar language to Hindi, written in a different yet related script), the standard orthography calls for a space between those two components. Yes, it's a different language. But I think it supports the idea of a mental separation between two "pieces".