You may know that Arabic has three grammatical cases: nominative, genitive, and accusative. There are two different issues at hand when discussing case. The first is what the different forms look like, and the second is what contexts are the different contexts they are used in. Let’s look at both of these very briefly.

What the forms look like

Let’s illustrate this with the easiest case, which would be a singular noun ending in a consonant, such as بيت bayt ‘house’. The three cases of the indefinite form (‘a house’) are these:

In informal Arabic, the endings -un (nominative) and -in (genitive) would normally not be pronounced, while -an (accusative) would be, since that one is spelled with a real letter rather than exclusively with diacritics.

And when this noun is made definite, such as by adding the definite article to it to make ’al-bayt ‘the house’, we get the following three forms, without the nunation, that is, without the n at the end of the suffix:

The endings -u, -i, and -a of these forms would not normally be pronounced in informal Arabic.

The forms just discussed are just one type of noun. The various case forms will look different for other types of nouns and adjectives, such as: words ending in a taa’ marbuuṭa, dual forms, masculine sound plurals, feminine sound plurals, diptotes, and nouns ending in a vowel or a hamza. We won’t discuss these here, but read on for resources on this at the end of this article.

Contexts for the three cases

We will only discuss the most basic contexts for the three cases here.

Nominative:

• The subject of a sentence is generally nominative.
  

  الْبَيْتُ هُنا.

  ’al-baytu hunaa.

  ‘The house is here.’

• A noun or adjective that is the predicate of a verbless sentence is always nominative, as in jamiil in the following sentence:
  

  الْبَيْتُ جَمِيلٌ.

  ’al-baytu jamiilun.

  ‘The house is beautiful.’

Genitive:

• The object of a preposition is genitive:
  

  في الْبَيْتِ

  fii l-bayti

  ‘in the house’

• The second term of an إِضافة ’iḍaafa (a “possessive construction”) is always genitive, as in baabu l-bayti ‘door of the house’ in this example:
  

  هذا بابُ الْبَيْتِ.

  haadhaa baabu l-bayti

  ‘This is the door of the house.’

  Note that the first term of the ’iḍaafa, which here is baab could be in any of the three cases, depending on the grammatical function of the phrase in the sentence. It is only the second term (here ’al-bayt) that is always genitive.

Accusative:

• The object of a verb is always accusative.
  

  أَرى بَيْتًا.

  ’araa baytan.

  ‘I see a house.’

• A noun or adjective that is the complement of kaana or one of her sisters is always accusative, as in the case of the adjective jamiil in this example:
  

  كانَ الْبَيْتُ جَمِيلًا.

  kaana l-baytu jamiilan.

  ‘The house was beautiful.’

There are still many other contexts we haven’t discussed, especially for genitive and accusative, but these are the most frequent and important contexts. Examples of other contexts include nouns following certain particles and preposition-like words and nouns used in combinations with numbers.

Resources

The purpose of this page was to give you a basic idea of Arabic’s three cases: the forms of a simple noun and some of the most basic contexts in which they are used. For more detail I can recommend the following two pages:

• Arabic learning resources: Arabic case system.
• The English-language Wikipedia page on إِعْراب ’iʕraab.