Learn French: Je n'ai déballé que les couvre-lits sur lesquels tu dors. - I only unpacked the bedspreads on which you sleep.

Couvre-lits is a compound noun made of a verb (couvre, from couvrir) and a noun (lit). The rule for compound nouns is that verbs never take a plural marker. The noun part usually takes an 's' in the plural if it makes logical sense for there to be multiple of that item.

Why do we use sur lesquels here instead of just que or où?

To say 'on which', you need the preposition sur. When using a preposition like sur, avec, or pour to refer back to a thing, you use a form of lequel. Since couvre-lits is masculine plural, the pronoun must match it, becoming lesquels. You can't use que because it cannot be the object of a preposition like sur.

I know seulement means 'only'. Why use ne ... que here?

Ne ... que is a very natural and common way to express 'only' in French, functioning similarly to 'nothing but' in English. The ne goes before the conjugated verb (ai) and the que goes immediately before the specific thing being restricted (les couvre-lits). At a B2 level, mastering this structure is key to sounding more fluent instead of always relying on seulement.

Since les couvre-lits is plural, shouldn't the past participle be déballés?