Charles Ellicott Commentary 2 Kings 2:10

"And he said, Thou hast asked a hard thing: [nevertheless], if thou see me when I am taken from thee, it shall be so unto thee; but if not, it shall not be so." — 2 Kings 2:10 (ASV)

You have asked a hard thing. —Because to grant such a petition was not in Elijah’s own power, but in God’s only. And therefore, in the next words, the prophet connects the fulfillment of his follower’s wish with a condition depending entirely upon the Divine will: “If you see me when I am taken from you, it shall be so to you” (Keil). ‘If the Lord thinks you worthy to witness my departure, you will be worthy to win your boon.’ Elijah thus disclaims power to fulfill the request. At the same time, it is implied that his departure will be something exalted above the perception of ordinary men” (Thenius).

When I am taken. —Literally, taken (participle pu’al, shortened form, as in Exodus 3:2; Isaiah 18:2).