Six Types of Derived Nouns

Comprehensive Final Revision: Root Classes, Morphological Changes, Conjugations, Nouns and Verb-Form Semantics

Each of the six derived-noun classes has a distinct semantic role.

ClassExampleRole
Active participleكَاتِبٌDoer
Passive participleمَكْتُوبٌPatient/result
Comparative/elativeأَكْبَرُGreater degree of a quality
Instrument nounمِفْتَاحٌTool
Time/place nounمَجْلِسٌPlace of the action
Resembling adjectiveحَسَنٌRelatively stable quality

مِفْتَاحٌ — مَجْلِسٌ
Translation: A key — a sitting place/assembly.

Pattern is evidence but contextual meaning is necessary for the final classification.