Learn Swedish: Attityden, den brukade vara så dålig. - The attitude, it used to be so bad.

This sentence uses left-dislocation to emphasize the topic. The noun Attityden is moved to the very front outside the main clause to grab attention, almost like a heading. Then, the pronoun den acts as a placeholder (a resumptive pronoun) taking the role of the actual subject in the sentence.

Why must we use den here rather than det?

The resumptive pronoun must always match the gender and number of the dislocated noun. Because attityd is an en-word (en attityd, attityden), you have to use den to point back to it.

Doesn't putting an extra noun before the subject break the V2 rule where the verb must come second?

It looks like it does, but it actually doesn't! In left-dislocation, the fronted noun (Attityden) is considered completely outside the main clause. The main sentence actually starts with den (position 1), making brukade the second element (position 2). The V2 rule remains perfectly intact.

What is the difference between saying brukade vara and just var?