Learn Dutch: Het was de burgemeester die het museum sloot. - It was the mayor who closed the museum.

This is a cleft sentence, a structure used to add strong emphasis. By splitting the sentence and starting with Het was... (It was...), you highlight exactly who performed the action. It tells the listener that it was the mayor specifically, rather than the city council or the director.

Why do we use die for 'who' here instead of wie?

In Dutch, the connecting word in a relative clause depends on the gender of the noun you are pointing back to. Because it refers back to de burgemeester, which is a de-word, we must use die. You only use dat for het-words. We avoid using wie here because wie is mainly used in questions or after a preposition (like 'aan wie'), not as a standard relative pronoun for subjects.

Why is the verb sloot placed at the very end of the sentence?

The second half of this cleft sentence, starting with die, acts as a subordinate clause. In Dutch, subordinate clauses always push the conjugated verb to the very end of the phrase.

What is the infinitive of sloot?

It comes from the irregular strong verb sluiten (to close). For a singular subject in the simple past tense, it becomes sloot. If the subject were plural, it would be sloten.