en_tn/rom/08/29.md

those whom he foreknew

"those whom he knew before he even created them"

he also predestined to be conformed

"he also planned in advance that they would be conformed"

to be conformed to the image of his Son

You can translate this in an active form. Alternate translation: "that he would change them to be like his Son" (See: rc://en/ta/man/translate/figs-activepassive)

Son

This is an important title for Jesus, the Son of God. (See: rc://en/ta/man/translate/guidelines-sonofgodprinciples)

that he might be the firstborn

"so that his Son would be the firstborn"

among many brothers

Here "brothers" refers to all believers, both male and female. Alternate translation: "among many brothers and sisters who belong to the family of God" (See: rc://en/ta/man/translate/figs-explicit)