Mark 9:23

There exists a neuter singular article τὸ at the beginning of the phrase that is apparently never translated by anyone. It is used here to nominalize the phrase Εἰ δύνῃ ("If you can") into a self-contained idea. This makes Εἰ δύνῃ not just a part of a broader sentence but a concept or point of focus that he is drawing attention to.

He is effectively quoting back the phrase "If you can" from the person's earlier words, treating it as an idea to respond to.

It's as if he is saying, "Let's talk about this 'If you can.'"

"If you can...."

"If you can!"

Τὸ Εἰ δύνῃ is nominalized to emphasize the phrase "If you can" as a significant concept or challenge being addressed. The article τὸ marks it as a distinct idea, drawing attention to its theological/rhetorical importance in the dialogue and therefore should not be overlooked.