自学印地语3 ： 在屋里

Why say that these words are oblique, when they haven't changed their form at all? Because in the singular, only some nouns change in the oblique. Masculine -a endings are the culprits here: they change to -e, as shown in the following list.

कमरा kamra    room
कमरे में  kamre men   in the room
लरका ladka    boy
लड़के से ladke se    by the boy
आगरे aagra         Agra
आगरे तक aagre tak    as far as Agra

Masculine -a adjectives describing oblique nouns change similarly:
छोटा कमरा
chota kamra     small room
छोटा कमरे में  
chote kamre men    in a/the small room

मोटा लड़का
mota ladka       fat boy
मोटा लड़के से mote ladke se     by the fat boy

मेरा बगीचा  
mera bagicha   my garden
मेरे बगीचे में  mere bagiche  men    in my garden

An -a adjective changes like this with all types of masculine noun..
So although  घर  ghar 'house' does not end in -a and therefore cannot change visibly in the oblique, -a adjectives qualifying it must change¡ all the same:

बड़े घर में
bade ghar men    in the big house
छोटे माकन में
chote makan  men    in the little house

Feminine nouns and adjectives are easier to deal with - they don't‘ change at all in the oblique singular:

छोटी मेज़ पर
choti mez par    on the little table
मेरी बहिन को
meri bahin ko     ti my sister.

छोटी मेज़ पर
choti mez par    on the little table
मेरी बहिन को
meri bahin ko     to my sister.

In the oblique plural, all nouns take the ending -o as shown:
मेज़
mez   table
मेजों पर
Mezo  par   on tables
कमरा
kamra   room
कमरों में
Kamro  men    in rooms