Learn Italian: Le scatole sono piccole, ma ne abbiamo riempite quaranta. - The boxes are small, but we filled forty of them.

Because of the pronoun ne. Usually, when using the helper verb avere (like abbiamo), the past participle ends in -o. But when you use ne to mean "of them," the past participle must agree with the gender and number of the noun that ne replaces. Since ne replaces le scatole (a feminine plural noun), riempito changes to riempite.

What exactly is ne doing in this sentence?

Here, ne translates to "of them." Whenever you mention a specific quantity of something you already talked about (like forty boxes), Italian requires you to use ne to replace the noun. You cannot just say abbiamo riempito quaranta; you need ne to connect the number back to the boxes.

Why does ne go before abbiamo instead of next to the number quaranta?

ne is a pronoun, and in Italian, this type of pronoun almost always goes immediately before the conjugated verb. Here, the conjugated verb is the helper verb abbiamo, so ne is placed right in front of it, leaving the quantity quaranta on its own at the end.

Why is the adjective piccole used instead of ?