A pronoun or demonstrative may serve as اسمُ مَا. It remains in its familiar form; the predicate takes the required form.
Pattern
Negative sentence
Meaning
pronoun + emphatic بِ
مَا هُمْ بِمُؤْمِنِينَ
They are certainly not believers
feminine plural pronoun
مَا هُنَّ أُمَّهَاتِهِمْ
They are not their mothers
addressed pronoun + emphatic بِ
مَا أَنْتَ بِمَجْنُونٍ
You are certainly not mad
demonstrative
مَا هَؤُلَاءِ مُنَافِقِينَ
These are not hypocrites