Learn Danish: Der festes meget på den offentlige strand hvis vejret er godt. - A lot of partying goes on at the public beach if the weather is good.

This is the impersonal passive. By combining the dummy pronoun der (there) with the s-passive form of the verb at feste (to party), it describes an action happening in general without specifying who is doing it. Literally, it translates to "there is partied."

Why is it den offentlige strand and not just den offentlig strand?

When you use a definite article like den (the) before an adjective, the adjective must take the definite ending -e. Because there is an adjective, you must use the separate article den instead of attaching the definite suffix to the noun like you would in stranden.

Why is the word for "good" spelled godt instead of god here?

The adjective god describes the noun vejret (the weather). Because vejr is a neuter word (an et-word), the adjective must agree with it by adding a -t to the end.

If we moved hvis vejret er godt to the beginning of the sentence, how would the word order change?

The main clause would have to invert its subject and verb to keep the verb in the second position (the V2 rule). It would become: Hvis vejret er godt, festes der meget på den offentlige strand.