Learn Finnish: Pidätkö sääntöä reiluna vai epäreiluna? - Do you consider the rule fair or unfair?

This is the essive case ending. When using the verb pitää to mean 'to consider', the adjective describing your opinion must be in the essive case. You can think of the -na ending as meaning 'as', so you are literally asking if they consider the rule 'as fair' or 'as unfair'.

Why is sääntö (rule) in the partitive case (sääntöä)?

In this specific opinion-expressing structure (pitää + partitive + essive), the thing you are judging or expressing an opinion about is the object of the sentence, and it must take the partitive case.

Doesn't the verb pitää usually mean 'to like'?

It does when it is followed by the elative case (-sta/-stä), like in pidän säännöstä (I like the rule). However, when pitää is followed by a partitive object and an essive adjective, the meaning changes to 'to consider' or 'to regard as'.

Why does the sentence use vai instead of tai for the word 'or'?

Both words mean 'or', but vai is used specifically in questions where you are asking the listener to choose one of the options. Because a rule cannot be both fair and unfair at the same time, and you want them to pick one, vai is the correct choice here.