The Five Nouns

Five inherited Arabic nouns can mark case with an entire long vowel rather than a final short vowel: أب “father,” أخ “brother,” حم “a relative by marriage,” فو “mouth,” and ذو “possessor of.” Under the right conditions, nominative uses ـو -ū, accusative ـا -ā, and genitive ـي -ī.

These forms are common in careful Modern Standard Arabic, especially أبوك، أخوه، ذو خبرة. They are not arbitrary alternate spellings. The long vowel encodes case, and it appears only when the noun meets strict construct-state conditions. This is a different system from the two-ending paradigm of diptotes.

The long vowels are case markers

With a second-person masculine suffix, أب displays all three forms:

The suffix -ka makes the noun construct: “father of you.” At sentence-final pause, the short vowel after k disappears, but the case-bearing long vowel remains audible.

All five paradigms

The genitive فيك “your mouth” is spelled like فيك “in you”; syntax disambiguates them. In contemporary prose, speakers often prefer ordinary فمك “your mouth,” but the special فو paradigm remains part of formal grammar.

The third-person suffix in فاه is -hu, whose vowel is lost at pause; the long ā before it remains the accusative marker.

The conditions are part of the rule

The special paradigm applies when the noun is:

1. singular, not dual or plural;
2. construct, either before an attached possessive suffix or before a following genitive noun;
3. not attached to first-person singular ي, which occupies the position needed for the visible contrast;
4. not a diminutive such as أُبَيّ or أُخَيّ;
5. in the case of فو, used without the م of ordinary فم;
6. in the case of ذو, used in the meaning “possessor of” and followed by its genitive complement.

If the conditions fail, use ordinary short-vowel or other relevant inflection.

Neither noun is construct, so the nominative is short -u, not أبو. The definite article does not satisfy the construct condition.

With first-person ي, spelling stays أبي across the cases, and case is estimated rather than expressed by alternating و، ا، ي:

The identical surface form does not mean the syntax has no case. The first is nominative and the second accusative; the possessive yāʾ prevents the difference from surfacing.

ذو is always relational

ذو cannot stand alone with the intended meaning “possessor.” It must introduce a genitive complement and agree with its possessor in gender and number. The singular masculine case forms are ذو، ذا، ذي; singular feminine is ذات with ordinary short-vowel case.

The gender of خبرة does not select ذات. ذات is feminine because the possessor مهندسة is feminine.

Number and plural class remain lexical

The five-noun rule covers special singular case forms. Dual and plural forms use their own paradigms:

إخوة and إخوان are both established plurals of أخ, with lexical and contextual preferences; neither is produced by the Five Nouns case rule. Human plurals take human plural agreement.

The nonhuman plural أفواه “mouths” instead takes feminine singular agreement:

A contemporary informational sentence

Both أخو and ذو are nominative construct nouns, so both display long ū.

Common Mistakes

Dropping the case-bearing long vowel

Choosing gender from the following noun

Inventing a regular plural

Combining two with a plural noun

Giving a nonhuman plural human agreement

Key Takeaways

• Under strict singular construct conditions, the Five Nouns mark nominative with ū, accusative with ā, and genitive with ī.
• Without construct state, and with first-person ي, the visible long-vowel alternation does not apply.
• فو requires loss of م; ذو must mean “possessor” and take a genitive complement.
• Duals, plurals, gender agreement, and human-versus-nonhuman agreement remain separate systems.