What is the difference between "what you could not bear patiently" and "what you could not bear patiently" in Surat Al-Kahf?

The exegetes mentioned that the form (تستطع - tastati') in His Almighty's saying: "I will inform you of the interpretation of that which you could not [تستطع] endure patience over" was used because the problem was heavy. Whereas the form (تسطع - tasti') came after the explanation and clarification to suit the lightness of the matter after the problem disappeared. As linguists say: an increase in the building [of a word] is an increase in the meaning, and the heavier is met with the heavier, and the lighter with the lighter.