Learn Croatian: Njegova kći je mlađa za dva mjeseca. - His daughter is two months younger.

In Croatian, you cannot just place the amount of difference directly next to a comparative adjective. To express exactly by how much someone or something differs, you must use the preposition za followed by the accusative case. It essentially translates to younger by two months.

Since the preposition za takes the accusative case, why does mjeseca look like it is in the genitive?

The entire phrase dva mjeseca acts as the accusative object of za. However, because the phrase starts with the number dva (two), the noun following it is forced into the genitive singular. The preposition governs the number phrase as a whole, but the number itself still dictates the internal case of the noun.

Why is the word for daughter kći here, when I often hear people say kćer?

kći is the proper nominative form, which is required here because the daughter is the subject of the sentence. The form kćer is technically the accusative case, used for direct objects. Although many native speakers casually use kćer as the subject in everyday colloquial speech, standard Croatian requires kći.

Could we move the verb je later in the sentence, such as Njegova kći mlađa je za dva mjeseca?