Reuben. For the order of the tribes see App-45.
live, and not die. Figure of speech Pleonasm (App-6) for emphasis, reversing Genesis 49:3; Genesis 49:4.
let not. The word "not" is rightly repeated from preceding clause, to supply the Figure of speech Ellipsis (App-6).
men. Hebrew. methim. men as opposed to women and children.