Learn Danish: Som hovedregel må man ikke ryge indendørs på arbejdspladsen. - As a general rule, you may not smoke indoors at the workplace.

Because Danish is a V2 (verb-second) language. The sentence starts with the adverbial phrase Som hovedregel, which takes the first position. The finite verb må must come second, pushing the subject man and the negation ikke to the third and fourth positions.

Doesn't må usually mean 'must'? Why does it translate to 'may' here?

In Danish, må can mean both 'must' (obligation) and 'may' (permission). When combined with ikke, the phrase må ikke almost always translates to 'must not' or 'may not' in the sense of something being forbidden. If you want to say that you 'do not have to' do something, you use behøver ikke instead.

Why is there no article before hovedregel?

Fixed phrases using som to describe a role, status, or classification often drop the indefinite article in Danish. Som hovedregel is a set expression meaning 'as a general rule', so no en is needed.

Why do we use the preposition på with arbejdspladsen instead of i?

Institutions, public places, and workplaces traditionally take the preposition på rather than i in Danish. You are considered to be at the function or location of the workplace, so it is på arbejdspladsen, similar to på kontoret (at the office) or på hospitalet (at the hospital).