Learn Italian: I portachiavi sono al pianterreno. - The key rings are on the ground floor.

Portachiavi is a compound noun made of a verb (porta, meaning carries or holds) and a plural feminine noun (chiavi, meaning keys). When a compound noun is formed by a verb and a plural noun, it is invariable, meaning its form doesn't change in the plural. You just change the article from il to i.

Why is the verb part (porta) conjugated in this specific way?

In Italian verb-noun compound words, the verb is almost always in the third-person singular form of the present tense. Think of il portachiavi as literally meaning 'it carries keys' or 'it holds keys'.

Is pianterreno also a compound noun? How would you make it plural?

Yes, pianterreno is made of the noun piano (floor) and the adjective terreno (ground). Notice how the final 'o' of piano drops to make pronunciation smoother. For compound nouns formed by a noun plus an adjective, both parts usually change in the plural. So, if a building somehow had multiple ground floors, the plural would be i pianiterreni.

Why do we use al instead of sul (on the) for 'on the ground floor'?