Learn Italian: Questa virtù è meno diffusa del coraggio. - This virtue is less widespread than courage.

You have to look at the words around it! The demonstrative adjective Questa (this) is feminine singular, the verb è (is) is singular, and the adjective diffusa (widespread) ends in -a, which is feminine singular. If it were plural, the sentence would start with Queste virtù sono meno diffuse.

Why does the sentence use del to mean "than"?

In Italian, when comparing two nouns or pronouns directly (in this case, virtù and coraggio), you generally use the preposition di for "than". Because the noun coraggio needs its definite article il, di combines with il to form the combined word del.

Is diffusa an adjective here?

Yes. It's actually the past participle of the verb diffondere (to spread or diffuse), being used as an adjective. Like all regular adjectives, it has to agree in gender and number with the noun it describes, which is why it takes the feminine singular ending -a to match virtù.

If I wanted to say "more widespread" instead of "less widespread", what would I change?