Learn Swedish: Det dansas och sjungs på karnevalen. - There is dancing and singing at the carnival.

This is called the impersonal passive. Intransitive verbs (verbs without a direct object) can take the -s passive ending to describe a general activity without specifying who is doing it. It focuses entirely on the action taking place.

Why do we need Det at the start of the sentence?

Because the impersonal passive removes the actual actors from the sentence, Swedish uses the dummy subject Det to fill the empty subject position. This ensures the sentence still follows standard Swedish word order, making it literally mean 'It is danced and sung'.

How exactly is the present tense passive formed for these verbs?

To form the present passive, you take the active present tense of the verb and replace the final -r with an -s. For example, dansar becomes dansas and sjunger becomes sjungs.

Does this mean exactly the same thing as Man dansar och sjunger på karnevalen?

They are very similar, but the impersonal passive (Det dansas...) puts the spotlight entirely on the festive atmosphere and the activities themselves. Using man (one/people) keeps slightly more focus on the unnamed individuals doing the actions.