When a masculine word ends in अ, इ, ई, उ, ऊ, it does not change in the plural :

So बाल (child) will be बाल (children)

When a masculine word ends in आ, change it to ए to make it plural :

काला (black singular) = कालो (black plural)

When a feminine word ends in अ or आ, change it to एँ :

रात (dark sing.) = रातों (dark plu.)

When a feminine word ends in या, change it to याँ :

कुटिया (shack) = कुटियाँ (shacks)

When a feminine word ends in इ or ई, change it to इयाँ :

बिल्ली (cat) = बल्लियाँ (cats)

When a feminine word ends in उ, ऊ, औ change it to एँ :

गौ (cow) = गौएँ (cows)

Some singular words don’t change in the plural :

मामा (uncle) = मामा (uncles)

There are some masculine words that don’t follow any rules when they change in the feminine form :

पिता (father) = माता (mother)