مَفْعُولٌ مُطْلَقٌ is a mansub verbal noun used to emphasize an action, describe its kind or quality, or state its number.
A verbal noun names the action without tying it to a person or time. For example, ضَرْبٌ means striking. In a sentence it appears as ضَرْبًا because the maf'ul mutlaq is mansub.
Recognition questions:
1. Is the word a verbal noun connected to the verb's action?
2. Does it emphasize, describe, or count that action?