"Afternoons with friends are fun."
Translation:Las tardes con amigos son divertidas.
I think that the plural "divertidas" is used because of "las tardes" but in English, fun is "fun" and never plural.
"Tardes" is a feminine noun so it has to have the article "las." Since the adjective "fun" is describing the "afternoon" it has to match gender and number ("divertidas").
Because it's describing "las tardes" which is feminine. It's the same principle as "un vestido bonito" vs. "una falda bonita," only this time the adjective isn't right next to the noun.
The "las" only refers to "tardes." I'm going to report my answer "Las tardes con los amigos son divertidas" because both the given translation, and mine are valid translations given different contexts. Mine suggests that I'm talking about a specific group of friends, while the given one is a more general blanket statement.
The rule that I have noticed seems to be, when talking about a general category of objects, you include the article only when the category is the subject of the sentence.
I think that is partly true - in that 'with friends' is a sort of restrictive adjectival phrase - but I have also seen : Los granjeros respectan los animales (DO with article). Then again .... 'Los osos comen pescado ( DO no article but pescado is not individual fish but 'fish meat' ) ... can a native speaker say whether 'los osos comen los peces' would be more correct than 'los osos comen peces'?
They are both valid translations, but would be used in different contexts. "Los osos comen peces" is a general statement saying that bears eat fish (think encyclopedia entry). "Los osos comen los peces" implies that there is a certain group/set of fish that the bears are currently eating. Imagine commentary on a nature show of grizzlies catching salmon out of the waterfall.