@relative clause

I don't have the space to explain here in detail how relative constructions work in Sanskrit. But I will give a couple examples.

mUSikaz carati मूषिकश्चरति "mouse walks"

mUSiko yaz carati मूषिको यश्चरति "mouse that walks"

yo mUSikaz carati यो मूषिकश्चरति "mouse that walks"

mUSikaz carati yaH मूषिकश्चरति यः "mouse that walks"

mUSikaz carati yaH मूषिकश्चरति यः, sa phalaM vindati स फलं विन्दति "mouse that walks, he finds a fruit"

carati yaH चरति यः, sa phalaM vindati स फलं विन्दति "who walks, he finds a fruit"

carati yo yaH चरति यो यः, sa sa phalaM vindati स स फलं विन्दति "whoever walks, he finds a fruit"

Points to remember —

yaH यः means "the one who"

muSiko yaH मुषिको यः means "the mouse who"

yam यम् means "the one whom"

muSiko yaH मुषिको यः means "the mouse whom"

yena येन means "the one by whom"

muSikeNa yena मुषिकेण येन means "the mouse by whom"

The fact that the headnoun and the /yad- take the same ending is horribly confusing at first. Why? When you hear muSikeNa मुषिकेण the first thought that comes to your mind is "by mouse", but if there is a yena येन nearby, muSikeNa yena मुषिकेण येन does not mean "by the mouse" — it means "the mouse by whom". Therefore —

muSikeNa yena phalaJ coritaM sa zuklaH |
मुषिकेण येन फलं चोरितं स शुक्लः ।

translates into —

"the mouse by which the fruit was stolen was white",
"the mouse that stole the fruit was white".

and not into anything that starts with "by the mouse".