Why does "zout" come before "gebruiken" here? I thought the verb had to come either first or second in a question.
Yes, and no: it has to be the first constituent (i.e. sentence part) in a closed question (aka 'yes/no question), or the second constituent in an 'open question (such as this one).
Hoeveel zout gebruiken jullie?
• hoeveel zout= one constituent, the Direct Object, which in a statement normally comes after the verb, but here it's fronted because of the interrogative adverb. (Well, to be entirely honest, I think it's an OD, but I'm not 100% sure of it)
• gebruiken= second constituent, verb.
• jullie= third constituent, Subject.
For further information on constituents, please check https://en.m.wikipedia.org/wiki/Constituent_(linguistics)
This question written in Dutch uses the plural form, but the English form has no indication that it is plural, so if it is translated back into Dutch from English, the singular will be used, which will be wrong. Doesn't English have a singular and plural form for the second person?