Here "sin" is described as a master that a slave would serve. Also, "sin" refers to the power that lives in us that makes us choose to do what is sinful. AT: "For you were slaves to the power of sin" (See: [[rc://en/ta/man/translate/figs-personification]] and [[rc://en/ta/man/translate/figs-metonymy]])
Here the word "heart" refers to having sincere or honest motives for doing something. AT: "but you truly obeyed" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Here "form" refers to the pattern or way of living that leads to righteousness. The believers change their old way of living to match this new way of living that Christian leaders teach to them. AT: "the teaching that Christian leaders gave you" (See: [[rc://en/ta/man/translate/figs-activepassive]]).