Reference
Which and who
"Which" and "who" take the ending their own clause needs, not the noun's.
What it is for
- Different pronouns for people and for things
Kusagil on keegi, kes sinust tõeliselt hoolib.
Somewhere there is someone who truly cares about you.
Bioloogia on teadus, mis uurib elu.
Biology is the science that studies life.
- Its ending depends on its job in its own part of the sentence
Ta on hea režissöör, kelle filme tasub vaadata.
He is a good director whose films are worth watching.
Tegin nimekirja asjadest, mida poest tuua.
I made a list of things to get from the store.