As with all Croatian cases, you need to know how to form the genitive case, and when it is used.
The genitive is the most important and useful of all Croatian cases, for the simple reason that it is the most commonly used. It is used after prepositions, numbers, quanities and when 'of' is used in English.
It is used after ima ... to mean 'there are some ...'.
Words like nekoliko (several), puno (lots of), mnogo (many) are followed by the genitive.
The genitive case is used in Croatian time related expressions, some of which can be seen in the holiday phrases under Days General.
jutro 'morning', and dan 'day' are masculine nouns, and večer 'evening' a feminine nown.
This is reflected in the different endings in the following phrases:
jednog jutra 'one morning'
jednog dana 'one day'
jedne večeri 'one evening'
The masculine singular endings are og, eg for adjectives and a for nouns.
The vast majority of genitive feminine singular endings are e for both adjectives and nouns.
Išla sam do njihove stare kuće 'I went to their old house'.
As well as being one sentences from common Croatian words, it shows a useful way to learn the genitive. This is to learn sentences which use the genitive as models for how and when the genitive is used, and to help memorise the appropriate endings.
Feminine nouns ending in a consonant (such as večer) add an i. E.g. večeri.
The idea isn't necesssarily to learn all these exceptions, but to be aware of them.
The masculine plural endings are ih for adjectives and a for nouns.
There are a few common exceptions, which behave like female plural nouns, ending in i.
sat 'hour' as in what time is it?, has a genitive plural of sati.
ljudi (people) is ljudi in the genitive plural.
The feminine plural endings are ih for adjectives and a for nouns.