Learn Croatian: Selidba će biti laka jer imamo kartonske kutije. - The relocation will be easy because we have cardboard boxes.

It comes from the verb seliti (to move or relocate). The suffix -idba is used to create nouns from verbs, which you might recognize from other words like plovidba (sailing) from ploviti.

I usually see lako used for 'easy'. Why is it laka here?

You often see lako (the neuter adjective or adverb form) in impersonal phrases like lako je (it is easy). However, in this sentence, the adjective directly describes the feminine noun selidba. Therefore, it must agree in gender and number, becoming the feminine singular laka.

What case is kartonske kutije in this sentence?

It is in the accusative case because it is the direct object of the verb imamo (we have). For feminine nouns that end in -a in the singular, the nominative plural and accusative plural are identical (both end in -e). That is why it looks exactly like the subject form.

Could I change the word order to Selidba bit će laka?