Learn Italian: Ho letto tutte le curiosità sulla città. - I read all the curiosities about the city.

In Italian, nouns that end in an accented vowel, like curiosità, are invariable. This means they do not change their endings in the plural. So, 'the curiosity' is la curiosità, and 'the curiosities' is le curiosità.

Since curiosità and città do not change in the plural, how can we tell which one is plural and which one is singular here?

We have to look at the words around them! For curiosità, the words tutte le (all the) tell us it is plural. For città, the combined preposition sulla (su + la) contains the singular feminine article la, telling us it is just one city.

Does tutte have to match curiosità even though curiosità does not change its ending?

Yes! Even though curiosità is invariable, it is still a feminine plural noun in this sentence. Therefore, the adjective must agree with it, which is why we use the feminine plural form tutte instead of tutti or tutto.

Why does the sentence use sulla for 'about the'? Doesn't su usually mean 'on'?