Learn Croatian: Ova breskva je slađa nego ona jabuka. - This peach is sweeter than that apple.

To form the comparative, the suffix -ja (for the feminine form) is added to the root. The fleeting a in the ending of sladak is lost, and the k drops out entirely. Finally, the d combines with the j from the suffix to form the letter đ, resulting in slađa.

Can I use od instead of nego to say "than"?

Yes, but the grammar changes. The conjunction nego keeps the second noun phrase in the same case as the first (nominative). If you use the preposition od, the following noun phrase must take the genitive case. The sentence would then be: Ova breskva je slađa od one jabuke.

Why is the second noun phrase ona jabuka in the nominative case rather than an object case?

The word nego links two grammatically parallel items. Because the first item being compared (Ova breskva) is the subject of the sentence and in the nominative case, the second item (ona jabuka) must also be in the nominative case to match it.

We have ova for "this" and ona for "that". When would I use ta?