Learn Finnish: Käytetty sohva oli liian painava kantaaksemme sen kierrätyskeskukseen. - The used sofa was too heavy for us to carry to the recycling center.

This is the core of the excessive resultative construction. It starts with the first infinitive kantaa (to carry), adds the translative marker -kse- (which expresses a result or purpose here), and finishes with the possessive suffix -mme (our). Combined with liian (too), the structure liian painava kantaaksemme literally means "too heavy for our carrying".

Does the possessive suffix on kantaaksemme require a separate pronoun in the sentence?

No, the "we" is entirely implied by the -mme suffix. You do not need a separate word for "us" (like meille). The sentence sets up the sofa as the subject, but the action of carrying belongs to the hidden "we" marked on the verb itself.

Why is the object sen (it) in the genitive-looking accusative case instead of the partitive?

Even though it is attached to an infinitive, normal object rules apply. The action of carrying the sofa to the recycling center is a complete, finished action, which rules out the partitive. Because there is a grammatical subject ("we") embedded in the -mme ending, the total object takes the genitive-like accusative sen rather than a nominative object.

I know kierrätyskeskukseen means "to the recycling center," but why does the ending look like -kseen instead of just adding -en?