Learn Italian: I dopocena in spiaggia sono indimenticabili. - The after-dinners on the beach are unforgettable.

Dopocena is a compound noun made of dopo (after) and the feminine noun cena (dinner). When a compound word is formed from an adverb/preposition plus a noun, and the new word takes a different gender than the original noun, it stays invariable in the plural. To make it plural, you only change the article to i.

Since cena is feminine, why does the combined word dopocena become masculine?

Compound words that combine dopo with an event usually become masculine nouns referring to "the time after" that event. You will also see this in common words like il doposcuola (after-school activities) or il dopoguerra (the post-war period).

Why do we use in spiaggia instead of sulla spiaggia for "on the beach"?

In Italian, many common locations use the preposition in without an article to describe being there for a typical activity, much like saying "in school" or "in bed" in English. While sulla spiaggia is grammatically fine (literally "on the sand/beach"), in spiaggia is the most natural way to say you are spending time at the beach.

How is the word indimenticabili built?