How did classical Arabic use case endings for subjects and objects?

Classical Arabic preserves an early Semitic case system where the ending of a noun or adjective indicates its grammatical function within a sentence. This system utilizes a tripartite set of suffixes to distinguish between the nominative, accusative, and genitive cases.1

For a typical noun, these endings function as follows:

• Nominative: Used for the subject of a sentence, the noun takes the suffix -un (e.g., kalb-un meaning "a dog" as a subject).1
• Accusative: Used for the direct object, the noun takes the suffix -an (e.g., kalb-an meaning "a dog" as an object).1
• Genitive: Used to indicate possession or following a preposition, the noun takes the suffix -in (e.g., kalb-in meaning "(of) a dog").1

While most nouns follow this pattern, some stems—known as diptotic stems—distinguish only between the nominative case and a combined accusative/genitive case. Although this declensional system is a hallmark of Classical Arabic, these case endings have largely disappeared from modern spoken dialects and are frequently omitted in spoken Modern Standard Arabic.12