en_tn/1ch/05/01.md

now Reuben

The word "now" is used here to mark the change from the lists of descendants to background information about Reuben. (See: rc://en/ta/man/translate/writing-background)

but his birthright was given to the sons of Joseph son of Israel

This can be stated in active form. Alternate translation: "but Israel gave Reuben's birthright to the sons of Joseph, another of Israel's sons" (See: rc://en/ta/man/translate/figs-activepassive)

Reuben had defiled his father's couch

This is a polite way to speak about Reuben sleeping with his father's secondary wife. The couch is the place where a man and his wife would have slept together. (See: rc://en/ta/man/translate/figs-euphemism and rc://en/ta/man/translate/figs-metonymy)

So he is not recorded as being the oldest son

This can be stated in active form. Alternate translation: "So the family history does not list Reuben as the oldest son" (See: rc://en/ta/man/translate/figs-activepassive)