Here, Paul refers to a believer as being one person before they believed in Jesus and a different person after they believed in Jesus. The "old man" refers to the person before he believes in Jesus. The person is spiritually dead and sin controls him. Paul describes our old sinful person as dying on the cross with Jesus when we believe in Jesus. AT: "our sinful person was on the cross with Jesus"
Paul compares the power sin has over a person to a master controlling a slave: a person without the Holy Spirit will always choose what is sinful. He is not free to do what pleases God. AT: "we should no longer be slaves to sin" or "we should no longer choose to do what is sinful" (See: [[rc://en/ta/man/translate/figs-metaphor]])