How is the acid HSO₄⁻ formed?
$\ce{HSO4-}$ look at the valencies of the hydrogen and sulphate ion, because of the numbers don't match i.e. hydrogen has a +1 charge that means it is deficient by 1 electron while sulphate has a -2 charge which means it has two extra electrons (it can also be viewed that it wants to 'give away' these two electrons) these valence numbers basically show the amount of bonds the molecules can form (since at this level of chemistry bonds can be seen as the sharing of electrons) thus the sulphate can form two while the hydrogen can only form one and in this case since it forms $\ce{HSO4-}$ that means there is one 'unhappy electron' or one that is unbonded since the bond between the sulphate and hydrogen makes hydrogen's charge neutral and takes an electron from the sulphate ion leaving the unbonded electron thus giving the molecule an overall charge of -1. Note that here I am just explaining the bonding that is occurring between the molecules but $\ce{H2SO4}$ DOES form I am just explaining the charges and values involved and also note that bonding and chemical reactions usually involve the VALENCE electrons!.

However this molecule ($\ce{H2SO4}$) will disassociate in water or solvent of some sort, and form ions as the person before me pointed out. into $\ce{H+}$ ions and $\ce{HSO4^-}$ thus forms the conjugate base of the acid ($\ce{H+}$ is the basis of most acidic reactions) 

If you're confused as to why $\ce{H2SO4}$ doesn't form, it does but $\ce{HSO4-1}$ is what happens after it disassociates in solution.

I don't know which part of the question or problem is where your confusion lies, hope I answered or helped!

(I am only a first year university student if my answer is wrong please notify me but it correlates with my notes, textbooks and what I was taught back in high school)