What is the ism of mā and lā resembling laysa?
لَيْسَ is one of the sisters of كَانَ: it raises its noun to rafa and puts its khabar into nasab.
Some uses of مَا and لَا negate in precisely that manner, and the grammarians say they resemble لَيْسَ — مُشَبَّهَتَانِ بِلَيْسَ. When they do, they work as it works.
In مَا زَيْدٌ قَائِمًا (mā zaydun qā'iman), Zayd is not standing, زَيْدٌ is the ism and stays marfū'; قَائِمًا is the khabar and is manṣūb.
They copy لَيْسَ exactly: the noun stays in rafa, the khabar drops into nasab.
The two are not interchangeable. مَا may enter upon a definite noun or an indefinite one; لَا is restricted to indefinite nouns alone — لَا رَجُلٌ أَفْضَلُ مِنْكَ (lā rajulun afḍalu minka), no man is nobler than you.
The resemblance is conditional, not automatic. Break the negation with إِلَّا, or put the khabar before the noun, and مَا stops governing — the sentence returns to being a plain mubtada' and khabar, both marfū'.
Because the conditions are fine and the usage is limited, this role is the least often met of the eight.1
Do not confuse this لَا with the لَا of absolute negation.
The لَا here resembles لَيْسَ: its noun stays marfū'.
The لَا لِنَفْيِ الْجِنْسِ does the opposite: its noun is manṣūb, and it carries no tanwīn — لَا رَجُلَ فِي الدَّارِ.
Same letter, two different jobs, opposite endings on the word that follows.
- It is the seventh of the eight in اَلْهِدَايَة فِي النَّحْو (al-Hidāya fī al-Naḥw) — the book that defines these two roles most clearly — and تَسْهِيل النَّحْو (Tasheel al-Nahw) takes its list and its order from there. ↩