en_tn/jer/13/23.md

Can the people of Cush change their skin color, or a leopard change its spots?

This rhetorical question is used to state an example of something that is impossible. This question can be written as a statement. Also, the understood verb "can" maybe be supplied in the second phrase. Alternate translation: "The people of Cush cannot change their skin color and a leopard cannot change its spots." (See: rc://en/ta/man/jit/figs-rquestion and rc://en/ta/man/jit/figs-ellipsis)

If so, then you yourself, although accustomed to wickedness, would be able to do good

This statement is ironic because the examples about the Cushite and the leopard are impossible situations. This states that if those impossible things could happen then they could do good. Alternate translation: "Just like these things are impossible, so it is also impossible for you who always do evil to do good" (See: rc://en/ta/man/jit/figs-irony)