What is a non-declining ism (mabnī)?

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.

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 mabnī. 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 mabnī, and so is every past-tense verb and every command.

The definition carries a qualifier that matters: a word is mabnī when its ending is fixed for a reason other than a weak letter. That is what separates هٰذَا, which is mabnī, from مُوسَى, which is mu'rab with a hidden i'rab.

Do not confuse a mabnī word with an ism whose i'rab is hidden.

هٰذَا is mabnī. 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 mabnī.