Learn Italian: Le metro urbane sono sempre affollate. - Urban subways are always crowded.

metro is short for metropolitana (subway). In Italian, nouns that are abbreviations or clipped versions of longer words usually do not change their endings in the plural. Since the original word is feminine, we use the feminine plural article le, but the noun itself stays metro.

If metro does not change its ending, how do we know this sentence is plural?

You have to look at the words around it! The article le, the verb sono, and the adjectives urbane and affollate are all in the feminine plural form. Even though an abbreviated noun does not change, the adjectives and articles connected to it still have to agree with its gender and number.

Wait, isn't metro a masculine word meaning "meter"?

Yes, but they are two different words that happen to look the same. Il metro (masculine) is the unit of measurement, and it has a normal plural: i metri. La metro (feminine) is the subway, and it has an invariable plural: le metro.

What is the base form of the word affollate?

The base dictionary form is affollato (crowded). Because it describes le metro, which is feminine plural, the ending changes to -e to become affollate.