Learn Italian: La colla che stai usando è più forte del cartone. - The glue that you are using is stronger than the cardboard.

When comparing two nouns using più ... di (more ... than), you usually need to combine di with the definite article of the second noun. Since cartone is masculine singular, it takes the article il. The preposition di + il combines to make del.

Why use "stai usando" instead of just "usi"?

stai usando is the present continuous tense, made with the verb stare plus the gerund (the -ando or -endo verb ending). It emphasizes that the action is happening right at this exact moment ("are using"). You could say usi ("use"), but stai usando is much more specific to right now.

Does the adjective "forte" change endings depending on the noun?

Yes, but because forte (strong) belongs to the group of adjectives that end in -e in their base form, it stays the same for both masculine and feminine singular nouns. Here, it describes the feminine noun la colla. If you were talking about plural items, it would change to forti.

Doesn't "che" usually mean "what"?

che is a very flexible word! When asking a question, it means "what" (like in Che cosa?). But it is also a relative pronoun used to connect phrases, where it means "that", "which", or "who". Here it connects "the glue" with the phrase "you are using".