Albert Barnes Commentary Matthew 17:10

"And his disciples asked him, saying, Why then say the scribes that Elijah must first come?" — Matthew 17:10 (ASV)

Why then say the Scribes, etc. The disciples appear to have been satisfied now that he was the Messiah. The transfiguration had taken away all their doubts.

But they recollected that it was a common doctrine among the Jews that Elijah should appear before the Messiah came; and they did not then recollect that he had appeared. To this difficulty the word then refers. "We are satisfied that you are the Christ. But Elijah is not yet come, as was expected. What, 'then,' is the meaning of the common opinions of our learned men, the Scribes? Were they right or wrong in their expectation of Elijah?" See Barnes on Matthew 11:14.