Why can't you form an acid salt from a monoprotic acid?
Monoprotic acids have only one H and that goes with the OH of a base to form water. To form an acid salt you need another H ion.
Example: Acetic acid, HAc, reacts with NaOH to form a salt + H2O.
HAc + NaOH ==> NaAc (sodium acetate) + H2O but you've used the only H in the acid.
Example: For H2SO4.
NaOH + H2SO4 ==> NaHSO4 + H2O. You see you have one H to form the H2O and that leaves the other H intact to form NaHSO4.