وَلَمَّا بَلَغَ أَشُدَّهُۥٓ ءَاتَيْنَٰهُ حُكْمًا وَعِلْمًا وَكَذَٰلِكَ نَجْزِى ٱلْمُحْسِنِينَ
"When he reached his full strength We gave him judgement and knowledge. Thus do We reward those who do good."
أَشُدّ — root ش-د-د: strength, firmness. Ashudd — the age at which the powers of body and mind are complete. The lexicographers differ over the singular of the word (whether shadd or shidda); since the disagreement does not change the meaning, I do not go into its detail.
Their coming indefinite is worth recording, and the lexicographers explain it as taʿẓīm (magnification): "a judgement and a knowledge" — that is, not a determined quantity but a share superior in kind.
حُكْم — root ح-ك-م: to make a thing firm, to put it in its proper place; from this, to judge. The root was treated at Luqmān 31/12-13 (wa-la-qad ātaynā Luqmāna'l-ḥikma).
And the order of the two words must be recorded: first ḥukm, then ʿilm. I give this as an observation; the sentence names the capacity to decide before knowledge.
وَكَذَٰلِكَ نَجْزِى ٱلْمُحْسِنِين — and the pattern occurs three times in the sūra: 22, 56 (lā nuḍīʿu ajra'l-muḥsinīn) and 90 (the same expression, in Joseph's mouth). This is a repetition verifiable within the sūra, and it will be treated at verse 90.