Thou hast 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 fulfilment of his follower’s wish with a condition depending entirely upon the Divine will: “If thou see me when I am taken from thee, it shall be so unto thee (Keil).” ‘If the Lord think thee worthy to witness my departure, thou wilt be worthy to win thy boon.’ Elijah thus disclaims power to fulfil 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).