Learn Danish: I 1970'erne var den stil populær. - In the 1970s, that style was popular.

To talk about a specific decade, we take the decade number (here 1970) and add an apostrophe followed by the definite plural ending -erne. It translates to 'the 1970s'. When reading it out loud, you say the full year as a plural compound: nittenhundredeoghalvfjerdserne (nitten-hundrede-og-halvfjerds-erne).

Why does the verb var come before the subject den stil?

Danish strictly follows the V2 (verb-second) rule in main clauses. Because the sentence starts with the time phrase I 1970'erne, the verb var must take the second position, pushing the subject den stil to the third position.

Why use den stil instead of just stilen here?

While stilen means 'the style', using the independent word den before the uninflected noun acts as a demonstrative pronoun, meaning 'that style'. It places emphasis on a specific style being discussed from a previous context.

Since the sentence uses the demonstrative den, why doesn't the adjective populær get a definite '-e' ending?

Because populær is not placed directly next to the noun (which would be 'den populære stil'). Instead, it is a subject complement placed after the verb var. In this position, the adjective only agrees with the gender and number of the subject. Since den stil is singular and common gender (an 'en' word), populær stays in its base form.