Learn Italian: Gli zaini sono piccoli e leggeri. - The backpacks are small and light.

They describe zaini (backpacks), which is a masculine plural noun. Regular Italian adjectives change their endings to match the noun they describe. Since the base words are piccolo (small) and leggero (light), their masculine plural ending is -i.

Why is the word for "the" Gli here instead of I?

In Italian, masculine plural nouns usually take the article i for "the". However, if the noun starts with the letter z (like zaini), a vowel, or s followed by another consonant, you must use gli instead.

How would I write this sentence in the singular ("The backpack is small and light")?

It would be: Lo zaino è piccolo e leggero. The noun and adjectives change their -i endings back to the singular masculine -o, and the special article gli changes to its singular form lo.

What if the sentence was about feminine nouns, like le borse (the bags)?

The adjectives would have to match the feminine plural noun. You would change the endings to -e, making the sentence: Le borse sono piccole e leggere.