Tafsir of Al-Kahf 18:67

(He said), meaning al-Khidr said to Musa, peace be upon them both: (Verily, you will not be able to have patience with me).

This is a negation of his ability to be patient with him in the most emphatic manner, as the particle inna (verily) is used for emphasis, and lan (will not) is used for a negation that is more emphatic than other forms of negation. He refrained from saying "you will not be patient" (lan tasbir) and used instead "you will not be able" (lan tastati'), which serves to negate patience through a demonstrative proof; for capability is that upon which the action depends, and therefore the negation of the action necessarily follows from the negation of the capability. He used the indefinite form for "patience" (sabran) in a context of negation, which denotes generalization, meaning: you will not have any patience with me at all, not even the slightest amount. He justified this by saying: