Questions & Answers about Het is de buurman die een affaire heeft.
Why does the sentence start with Het instead of Hij if we are talking about the male neighbor?
This is a cleft sentence, which is a structure used to put emphasis on a specific part of a sentence. Just like in English ('It is the neighbor who...'), Dutch uses the dummy subject het to introduce the focus, regardless of whether that focus is a person, a plural noun, or a thing.
Since we are talking about a person, why do we use die instead of wie for 'who'?
English uses 'who' specifically for people, but Dutch simply uses its standard relative pronouns: die for de-words and plurals, and dat for het-words. Because de buurman is a de-word, we use die. You generally only use wie as a relative pronoun when it directly follows a preposition, such as in de buurman met wie ik sprak (the neighbor with whom I spoke).
Why does heeft go at the very end of the sentence?
The phrase die een affaire heeft is a relative clause, which functions as a subordinate clause. In Dutch subordinate clauses, the finite verb (heeft) gets pushed to the very end of the clause.