RobH_en_tn/num/19/13.md

That person must be cut off

Here the phrase "be cut off" means to be disowned and sent away. See how you translated this in Numbers 9:13. Alternate translation: "That person must be sent away" or "you must send that person away" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-activepassive)

the water for impurity was not sprinkled on him

This can be stated in active form. Alternate translation: "no one sprinkled the water for impurity on him" (See: rc://en/ta/man/translate/figs-activepassive)

the water for impurity

"the water that is sprinkled on impure things to make them pure" or "the water for making things pure"

He will remain unclean; his uncleanness will remain on him

These two phrases mean basically the same thing and are combined for emphasis. (See: rc://en/ta/man/translate/figs-parallelism)