What is the difference between the verse, "ما لم تستطع عليه صبراً" (what you could not endure patiently), and the verse, "ما لم تسطع عليه صبرا" (what you could not endure patiently)?

Ibn Kathir said in his Tafsir (exegesis) that the verse: (That is the interpretation of what you could not endure patiently) means the explanation of what you did not bear patiently. And when Al-Khidr explained it to him, he used the word "tasta'" (تسطع - could), whereas before that he said: "I will inform you of the interpretation of what you could not endure patiently," so he made the heavier word phonetically correspond to the heavier meaning, and the lighter word to the lighter meaning. An example of this is the verse: "So they could not [lasta'u - لَمْ يَسْطَاعُواْ] ascend it," meaning to climb over it, and His saying: "And they were not able to penetrate it [wa ma istaṭā'u lahu naqbā - وَمَا اسْتَطَاعُوا لَهُ نَقْبًا]," which is more difficult.