وَمِن قَبْلِهِۦ كِتَٰبُ مُوسَىٰٓ إِمَامًا وَرَحْمَةً وَهَٰذَا كِتَٰبٌ مُّصَدِّقٌ لِّسَانًا عَرَبِيًّا لِّيُنذِرَ ٱلَّذِينَ ظَلَمُوا۟ وَبُشْرَىٰ لِلْمُحْسِنِينَ
"Before it there was the Book of Moses (Mūsā), as a leader and a mercy. And this is a confirming Book, in the Arabic tongue: to warn those who do wrong, and as good news for those who do good."
إِمَامًا — root أ-م-م: to go in front; the direction intended. Imām — the one who goes ahead, the one by looking at whom others walk. The word's sense of road and direction comes from this root.
That the Book is qualified as a "leader" is worth recording: the Book is named not as a source of knowledge but as something that walks in front.
And the verse is part of the answer given to the demand in the fourth verse. There bi-kitābin min qabli hādhā — "a Book before this" — had been asked for. Here that Book is named, and the relation of this text to it is stated: muṣaddiq — confirming.
لِّسَانًا عَرَبِيًّا — the core of the root ع-ر-ب, "clarity, not concealing what one means", was analysed in al-Wāqiʿa at 56/37 (ʿurub); I rely on that treatment. The name of the language is named together with the sense of clarity in its root.
And the last two words of the verse separate two distinct tasks: li-yundhira (to warn) and bushrā (good news). The same text does two distinct jobs according to whom it addresses.