أَتَدْرُونَ مَا الْغِيبَةُ؟
“Do you know what backbiting is?”
The initial أَ introduces a question; تَدْرُونَ is a second-person plural present-tense verb, and مَا is interrogative.
قِيلَ: أَفَرَأَيْتَ إِنْ كَانَ فِي أَخِي مَا أَقُولُ؟
“It was asked: what if what I say is actually present in my brother?”
قِيلَ is a passive past-tense verb, “it was said.” إِنْ كَانَ begins a condition, answered by فَقَدِ اغْتَبْتَهُ. The following clause uses the contrasting condition to identify a false accusation.
بَهَتَّهُ means “you falsely accused him.” Grammar here supports the moral distinction; a lengthy independent grammar lesson is unnecessary.