quis

quis asks "who?" and quid "what?". The rest of the paradigm is shared with the relative: genitive cuius, dative cui, accusative quem (neuter quid), ablative quo and qua. The case is chosen by the word's job in its own clause, exactly as for a noun, which is why "whose book" is cuius liber and "to whom" is cui.

After si, nisi, ne and num it drops the question and means "anyone, anything" instead. The tag every Latin class learns is "after si the ali- falls away": si quis is si aliquis with the prefix gone. Keep it apart from the relative qui, quae, quod, which introduces a clause describing a noun you have already named.