Learn Italian: Queste scatole sono piene.

Queste is the feminine plural form of questo (this). We use it here because it must match the noun it describes, which is scatole (boxes) — a feminine plural noun. If the noun were masculine plural, you would use questi.

What is the singular form of queste scatole?

The singular form is questa scatola (this box). Because scatole is a feminine plural noun ending in -e, its regular singular form ends in -a. The word for "this" changes from queste back to the feminine singular questa.

Why does the adjective piene also end in -e?

In Italian, adjectives must agree in gender and number with the noun they describe. The dictionary form of "full" is pieno. Because scatole is feminine plural, the adjective changes its ending to match, becoming piene.

Queste, scatole, and piene all end in -e. Is that a coincidence?

It is not a coincidence! This matching effect happens because regular feminine words ending in -a change to -e in the plural. The demonstrative adjective (queste), the noun (scatole), and the descriptive adjective (piene) all take the same feminine plural ending. This "rhyming" is a great way to double-check your grammar!