Learn Italian: Le auto elettriche sono tanto costose quanto le moto. - Electric cars are as expensive as motorcycles.

Auto is an abbreviation of the feminine noun automobile. In Italian, abbreviated words don't change their endings in the plural. Since it is feminine, the plural uses the feminine article le, giving us le auto.

The same applies to le moto, right? What is it short for?

Exactly! Moto is short for motocicletta (motorcycle). Like auto, it is a feminine noun that keeps its singular ending in the plural, so it becomes le moto.

If auto doesn't change its ending, why do the adjectives elettriche and costose change?

Even though auto is invariable (its spelling doesn't change), it is still grammatically a plural feminine noun here. Articles and adjectives must always agree with the noun's true gender and number. That is why we use the feminine plural adjectives elettriche and costose.

Why does elettriche have an 'h' in it?

The singular feminine form is elettrica. When making a feminine adjective ending in -ca plural, we add an h to keep the hard 'c' (k) sound. So, elettrica becomes elettriche.