en_tn/gen/49/03.md

the beginning of my strength

Hebrew parents would dedicate their first child to God. The words "beginning" and "first-fruit" means that Reuben belonged to God in a special way. Alternate translation: "the first-fruit of my manhood."

outstanding in dignity, and outstanding in power

"You are first in honor and power."

Uncontrollable as rushing water

Jacob compares Reuben to water in a strong current to emphasize that he cannot control his anger and he is not stable. (See: rc://en/ta/man/translate/figs-simile)

you shall not have the pre-eminence

"you shall not be first among your brothers"

because you went up to your father’s bed. Then you defiled it; you went up to my couch.

This refers to when Reuben slept with Bilhah in Genesis 35:21. This can be made explicit: "because you went to my bed and had sex with Bilhah my concubine. You brought shame on me." (See: rc://en/ta/man/translate/figs-euphemism and rc://en/ta/man/translate/figs-explicit)