Nouns, adjectives, and pronouns all change their form depending on what’s happening to them in a sentence. There are four such cases in Icelandic:
These cases are always shown in the same order. Some words, like “lamb”, don’t change that much:
But other words, like “cat”, change a lot:
1st caseköttur
2nd casekött
3rd caseketti
4th casekattar
The first case is the default case and is the one that is shown in dictionaries.

Comparison to English

The Icelandic cases can be compared to the different forms that English pronouns have. In the sentence “He took his book with him”, the word “he” appears three times, but is different each time, indicating what function the word has in the sentence. “He” is analogous to the first case in Icelandic, “him” is analogous to both the second and the third case, and “his” is analogous to the fourth case.

Further reading