How does the dual (muthannā) show its i'rab?
Arabic does not say “two Muslims” by counting. It changes the word itself: مُسْلِمَانِ, one word meaning two.
And it does not mark its state with a vowel. It marks it with a letter:
| State | Sign | Example | |
|---|---|---|---|
| rafa | ـَانِ | مُسْلِمَانِ | muslimāni |
| nasab | ـَيْنِ | مُسْلِمَيْنِ | muslimayni |
| jarr | ـَيْنِ | مُسْلِمَيْنِ | muslimayni |
Nasab and jarr share one form, so two of the three states look alike — as they do for the sound plurals too.
Seven categories that express i'rab their own way — by letters instead of vowels, or by not showing it at all.
The ن on the end is not part of the word. It is the sign of the i'rab, and it drops away the moment the dual becomes a مُضَاف: مُسْلِمَا مِصْرَ, not مُسْلِمَانِ مِصْرَ.
Do not read the kasra on the nūn as jarr.
جَاءَ الْمُسْلِمَانِ — the word ends in a kasra, and it is marfū'. The alif is the sign; the nūn and its kasra are just the tail.
The rule that saves you: for the dual, look at the letter before the nūn. Alif means rafa. Yā' means nasab or jarr.