Comparatives are forms of adjectives used to express not a property (e.g. big) but a relative property (e.g. bigger). A comparative is again an adjective. The base form (big) is sometimes called positive.
In Croatian, most adjectives form their comparatives by adding -iji to their dictionary form or case-base:
dosadan (dosadn-) boring → dosadniji more boring
jeftin cheap → jeftiniji cheaper
pažljiv careful → pažljiviji more careful
sretan (sretn-) happy → sretniji happier
star old → stariji older
Comparatives formed in this way usually have the stress on the syllable that comes right before the added -iji (e.g. paž-lji-viji).
Unfortunately, most one-syllable adjectives have complicated alternations of their final consonants in comparative, so their comparative form must be remembered (or you can remember the patterns from the adjectives listed below, but you still have to remember which adjectives have alternations and which just add -iji):
brz fast → brži
glup stupid → gluplji
Some one-syllable adjectives use the simpler way to create comparatives and just add -iji. Often used ones are:
Many two-syllable adjectives, especially ones ending in -ak or -ok, undergo an even greater transformation: they get shortened (e.g. lose -ok) and then go through the consonant alternation as most one-syllable adjectives:
dalek distant → dalji
širok wide → širi
Few adjectives have completely irregular comparatives (everything so far was kind of regular, believe it or not):
dobar good → bolji
loš bad → gori
There's an adjective malen with more or less the same meaning as mali small; both use the same comparative. The same goes as mek, a bit archaic variant of mekan soft. The opposite happens to loš bad — it has a regular comparative and an irregular one! Both are used. The same happens to dug long, where two comparatives exist, dulji and duži (some people claim there's a small difference in meaning, but they are used with the same meaning in the spoken language).
Comparatives always have the obligatory -i in nominative sing. masc., and in cases where there's a choice of two endings in masc. and neut. genders (i.e. o vs. e) they always have e in endings, even širi wider.
To say that something is e.g. bigger than something else, use od + G after the comparative:
Damir je stariji od Ane. Damir is older than Ana.
We know now to say that something is bigger, but what about much bigger? It turns out that with comparatives you can use the adverbs of quantity (already introduced in 45 Quantities and Existence):
Tvoj auto je mnogo brži. Your car is a lot faster.
Damir je dosta stariji od Ane. Damir is quite older than Ana.
While mnogo sounds a bit formal when expressing relative quantities in Croatian (e.g. mnogo ljudi many people), it's very often used with comparatives even in very colloquial contexts.
If you want to express a difference, you can put the difference in accusative before the comparative, or use za¨ + A after the comparative:
More je stupanj toplije. The sea is warmer by one degree.
More je toplije za stupanj. (the same meaning)
To ask about how is something comparing to something else, use koliko how much; the comparative is not moved:
Koliko je Damir stariji od Ane? How much older is Damir than Ana?
— Dosta. Quite older.
— Osam godina. Eight years older.
There are two more forms that can be derived from any adjective that has a comparative. Both are quite simple to make — just a prefix needs to be added. This table summarizes them:
premlad too young|
najmlađi the youngest|
In the standard stress scheme, the stress moves to the prefix, while in the 'western' scheme it usually stays on its original place.
There's not much else to be told about these forms; you can specify what group you meant with the superlative, using od¨ + G:
Igor je najmlađi od njene djece. Igor is the youngest of her children.
You can also specify the context of the absolute superlative, what for is someone too something:
Igor je premlad da vozi auto. Igor too young to drive a car.