What is a non-declining ism (mabni)?
In one line
Definition — al-bina'
Key words in the Arabic
Understand it
هٰؤُلَاءِ ends in a kasra when it is the doer, when it is the object, and when it follows a preposition. Nothing moves.
Such a word is مَبْنِيّ, and what it has instead of i'rab is a position: we say it is in the position of rafa, or nasab, or jarr — فِي مَحَلِّ رَفْعٍ, and so on.
The three primary categories
Every ism falls into one of three: fully declining, partly declining, or non-declining. The category decides how the i'rab is expressed — not what the i'rab is.
All pronouns are mabni. So are the demonstratives (except the duals), the relative pronouns (except the duals), the question words, the conditional words, and the compound numbers eleven to nineteen. Every حَرْف is mabni, and so is every past-tense verb and every command.
The definition carries a qualifier that matters: a word is mabni when its ending is fixed for a reason other than a weak letter. That is what separates هٰذَا, which is mabni, from مُوسَى, which is mu'rab with a hidden i'rab.
How to spot it
In the Qur'an
Don't confuse it with hidden i'rab
Do not confuse a mabni word with an ism whose i'rab is hidden.
هٰذَا is mabni. Its ending is fixed by the nature of the word itself, and it never had an i'rab to show.
مُوسَى and الْفَتَى are mu'rab. They do have an i'rab in all three states — it simply cannot be pronounced on a final alif, so it is understood rather than heard.
The classical definition draws the line with four words: لِغَيْرِ اعْتِلَالٍ — fixed for a reason other than a weak letter. If a weak letter is the reason, the word is not mabni.
Related terms
Common questions
If a mabni word never changes, how do I do i'rab on it?
By naming the role and adding that the word is in the place of that case. You still say what it is doing — doer, object, possessed — and then record that the ending cannot show it because the word is built.
Next in the guideاَلْأَسْمَاءُ الْخَمْسَةُWhat are the five nouns (al-asma' al-khamsa)? →Updated