وَمَا كُنتَ تَتْلُوا۟ مِن قَبْلِهِۦ مِن كِتَٰبٍ وَلَا تَخُطُّهُۥ بِيَمِينِكَ إِذًا لَّٱرْتَابَ ٱلْمُبْطِلُونَ
"You were not reciting any book before this, nor were you writing one with your right hand. Had it been so, those bent on falsehood would surely have fallen into doubt."
تَخُطُّهُ بِيَمِينِك — khaṭṭ: to draw a line, to write. And the note bi-yamīnik (with your right hand) gives the concrete picture of the act of writing.
I record this as an observation: the verse does not leave the claim to an abstract debate about knowledge — it makes a proof of a life that passed before the eyes of the one addressed. Indeed the validity of the proof rests on the existence of those who knew that life.
إِذًا لَّٱرْتَابَ ٱلْمُبْطِلُون — irtiyāb (to fall into doubt) and mubṭilūn (those bent on falsehood).
The conditional structure of the sentence must be recorded: idhan — "had it been so". That is, the verse says that no ground exists that could have come into the other side's hands.