المُعْرَبُ والمَبْنِيُّ · المَمْنُوعُ مِنَ الصَّرْفِ · الأَسْمَاءُ الخَمْسَةُ

I‘rāb is the change on the end of a word that shows its job in the sentence. Depending on their role, most nouns move between three cases:

The default signs are ḍamma ـُ (nominative), fatḥa ـَ (accusative), and kasra ـِ (genitive). But not every noun follows this cleanly — and that is what this lesson is about.

The triptote (munṣarif) is the “normal” noun. It accepts all three case endings and takes tanwīn when indefinite. In the genitive it takes kasra.

Note the double vowel (tanwīn) on the indefinite forms, and the clean ḍamma / fatḥa / kasra progression. The diptote breaks two of these rules.

A diptote is a declinable noun with two special restrictions. This is the heart of the lesson.

The categories of diptotes

If a noun matches one of these, it is a diptote. (Proper names cover several rows.)

Diptotes in the Qur’an

Watch the ending in the genitive slot — fatḥa when bare, kasra when اَلْ or iḍāfa is present.

A mabnī noun keeps one form in every position — its ending is “built in” and never changes for case. These are about 10% of nouns.

* The dual demonstratives and relatives are the exception — they are declinable: هَٰذَانِ / هَٰذَيْنِ and اللَّذَانِ / اللَّذَيْنِ.

Mabnī nouns in the Qur’an

Five special nouns — أَبٌ · أَخٌ · حَمٌ · فُو · ذُو (“father, brother, father-in-law, mouth, possessor-of”) — show their case with a long letter instead of a short vowel, on one condition: they must be singular and muḍāf (not muḍāf to the ي of the speaker).

Still declinable (mu‘rab), but their final letter can’t carry a visible vowel, so the case ending is estimated (مُقَدَّر).

Note: some teaching books group these among the “unchanging” nouns because they never visibly move. Technically they are declinable with an estimated ending — which is why the three case slots above look identical. The full manqūṣ paradigm is in the Chart tab.