en_tn/exo/21/09.md

designates her as a wife for his son

"decides that she is the one who will be his son's wife"

he must not diminish her food, clothing, or her marital rights

This can be stated in a positive form. AT: "he must give the first wife the same food, clothing, and marital rights she had before" (See: rc://en/ta/man/translate/figs-doublenegatives)

must not diminish her food

"must not take away her food" or "must not give her less food"

or her marital rights

This includes things that a husband must do for his wife, including sleeping with her. AT: "and he must continue to sleep with her as before" (See: rc://en/ta/man/translate/figs-euphemism)