How do you spell that?

How many typical mistakes can be seen in the records of different people! One such error is the writing of the expression "what". How do you spell that? It is written separately and in this form: whether that.

Why is that? Remember that the particle "li" with all words is always written separately, so "what" is written and not otherwise. Nothing, chtoli, chtoli, stoli, etc. should not be! Only "what?".