who	to refer to people
which	to refer to things and concepts
who/which	to refer to animals when we want/don't want to give them a personality
whom	a formal word for who - must be used directly after a preposition
that	a more informal word for; who, which, when, where, why - ONLY USED IN DEFINIGN RELATIVE CLAUSES
when	to refer to time
where	to refer to place or situation
why	often after the word reason (=the reason for which) - ONLY USED IN DEFINING RELATIVE CLAUSES
whose	the possessive of who and which; can also come after a preposition
what	=the thing(s) which - ONLY USED IN DEFINING RELATIVE CLAUSES
put the preposition before the pronoun	When we use a prepostion with a relative pronoun, it is more informal to...
This is the medical encyclopaedia to which I referred	To jest encyklopedia medyczna, do której się odnoszę (very formal)
This is the medical encyclopaedia which I referred to	To jest encyklopedia medyczna, do której się odnoszę (less formal)