Learn Finnish: Me liimaamme vanhat pahvit laatikoiksi. - We glue the old cardboards into boxes.

The base word is laatikko (box). To say 'into boxes,' we add the plural marker -i- and the translative case ending -ksi, making the suffix -iksi. Adding this suffix closes the preceding syllable, creating -koik-. In Finnish, a closed syllable triggers consonant gradation, which forces the strong grade double kk to weaken to a single k, resulting in laatikoiksi.

Why are vanhat pahvit in the plural nominative form rather than the partitive?

In this sentence, the plural suffix -t actually marks the plural accusative (often called the total object). This indicates that the action is bounded and complete: we are using up all of these specific pieces of cardboard to finish creating the boxes. If the gluing was an ongoing, incomplete process, we would use the partitive case (vanhoja pahveja).

Is it the verb liimata that requires the translative case for the boxes?

Yes. Verbs that describe transforming, building, or making something into a new state or object naturally pair with the translative case to show the end result. Since the action of gluing transforms the cardboard 'into' boxes, the result must take the translative ending.

The word pahvi translates to 'cardboard', which is usually uncountable. Why is it pluralized as pahvit here?

While pahvi does refer to the uncountable material of cardboard, Finnish frequently uses the plural form pahvit to refer to individual, countable pieces, sheets, or broken-down flats of cardboard.