Hindi nouns have two genders (masculine and feminine) and three cases :
- nominative
- oblique
- vocative.

Singular
In singular, only some nouns of the masculine gender change according to the case. These are masculine nouns ending in -ā or -ā̃.
They change -ā to -e and -ā̃ to -ẽ in the oblique and vocative case :
लड़का laRkaa , कुआं kuā̃ > लड़्के laRke, कुएं kuẽ
There are exceptions even among them, like राजा raajaa, पिता pitaa, चाचा caacaa.
Similar change occurs also in adjectives, where the ending -ā is very frequent.