Elias verily cometh first, and restoreth all things, and how it is written That is, And, he told them, how it is written. As if he had said, Elijah's coming is not inconsistent with my suffering. He is come; yet I shall suffer. The first part of the verse answers their question concerning Elijah; the second refutes their error concerning the Messiah's continuing for ever.
Related Commentaries of Mark 9:12
Mark 9:12-13
12 And he answered and told them,Elias verily cometh first, and restoreth all things; and how it is written of the Son of man, that he must suffer many things, and be set at nought.
13 But I say unto you,That Elias is indeed come, and they have done unto him whatsoever they listed, as it is written of him.