The Five Nouns (الأسماء الخمسة) are a special group of nouns that show their case endings through long vowels (letters) rather than the usual short vowel marks (damma, fat-ha, kasra). They are: أبٌ (father), أخٌ (brother), حمٌ (father-in-law), فو (mouth), and ذو (possessor of).
When these nouns meet specific conditions, their case markers become: و (waw) for the nominative (مرفوع), ا (alif) for the accusative (منصوب), and ي (yaa) for the genitive (مجرور). For example: جاء أبوك (your father came — nominative with waw), رأيتُ أباك (I saw your father — accusative with alif), مررتُ بأبيك (I passed by your father — genitive with yaa).
The conditions for this special declension are: (1) the noun must be singular (not dual or plural), (2) it must not be diminutive, and (3) it must be in an إضافة (possessive construction) with something other than ياء المتكلم (the first-person possessive pronoun). When any condition is not met, the noun reverts to regular declension with short vowels.
The Five Nouns are often one of the first topics in Nahw that introduces learners to the concept of إعراب with letters instead of vowel marks. This same principle appears again with the sound masculine plural (declined with ـونَ/ـينَ) and the dual (declined with ـانِ/ـيْنِ). Understanding the Five Nouns gives you a template for grasping these related concepts.