What is a maqṣūr ism, and why does it never change?
الْفَتَى، الْهُدَى، الْعَصَا، الْمُصْطَفَى، مُوسَى، الضُّحَى — every one ends in a permanent alif, and every one looks identical in all three states.
| State | Sign | Example | |
|---|---|---|---|
| rafa | ḍamma | جَاءَ الْهُدَى | jā'a al-hudā |
| nasab | fatḥa | رَأَيْتُ الْهُدَى | ra'aytu al-hudā |
| jarr | kasra | بِالْهُدَى | bi-l-hudā |
Nothing moves. All three signs are estimated on the final alif, which cannot carry a vowel at all.
Seven categories that express i'rab their own way — by letters instead of vowels, or by not showing it at all.
The i'rab has not vanished. It is مُقَدَّرَة — estimated: present in the grammar, impossible to pronounce. The books name the impediment exactly: التَّعَذُّر (al-ta'adhdhur), sheer impossibility. An alif cannot carry a vowel at all.
So the parsing is written out in full: fā'il in rafa, its sign an estimated ḍamma on the final alif, prevented from appearing by impossibility.
A maqṣūr ism is not mabnī, however much it looks it.
هٰذَا never changes because that is simply what the word is. It is مَبْنِيّ, and it has no i'rab to show.
مُوسَى never changes because an alif physically cannot carry a vowel. It is مُعْرَب — it *has* an i'rab in each state; the alif just cannot pronounce it.
The classical definition of binā' rules maqṣūr out in four words: لِغَيْرِ اعْتِلَالٍ, fixed for a reason *other than* a weak letter. Here the weak letter is the whole reason.