sentences:  The difference between those two problems is that in QBF the size of the problem is the size of the formula and elements are just boolean variables, whereas in FO the size of the problem is the size of the structure and the formula is fixed.This is similar to Parameterized complexity but the size of the formula is not a fixed parameter.Disregarding empty structures, every formula is equivalent to a formula in prenex normal form (where all quantifiers are written first, followed by a quantifier-free formula).