Learn Swedish: Vem sjöng på bröllopet?

The infinitive is sjunga. It belongs to Group 4, which are strong verbs that do not take a past tense ending like -de or -te. Instead, they change their root vowel. For this verb, the u in sjunga changes to ö, giving us sjöng meaning sang.

Why do we use på for at the wedding?

In Swedish, the preposition på is commonly used for events, ceremonies, and gatherings. Since a wedding is a planned event you attend, you use på, just like you would say på festen for at the party or på konserten for at the concert.

Why is the word for wedding written as bröllopet?

That is the definite form, meaning the wedding. The base word is an ett word: ett bröllop. To make an ett word definite, you add the -et ending to the noun, making it bröllopet.

Why is there no helper verb like did in this question?

Unlike English, Swedish does not use a helper verb like do or did to ask questions. You simply use the past tense of the main verb. Because Vem meaning Who is the subject of the sentence, the word order stays straightforward: Subject (Vem) plus Verb (sjöng).