An unvowelled nun is a written nun with sukun: نْ. Tanwin is the double vowel sign at the end of a word; during connected reading it produces a sound like an unvowelled nun even though no separate nun is written. This phonetic relationship gives them the same four following-letter rules.
«مِنْ» — a written still nun.
«عَلِيمٌ» — dammatan, sounding like a final n in connection.
«عَلِيمًا» — fathatan; stopping has separate rules.
Inspect the next consonant for throat clarity, assimilation, conversion, or true concealment. Tanwin normally ends a noun; still nun may occur within a word. This distinction explains later exceptions.