Hi, Here’s a quick breakdown: 1. Someone married once. → OK, but sounds incomplete. Better as “Someone got married once.” 2. If I could go back, I would never marry. → OK, means you would avoid marriage in general. 3. If I could go back, I would never have married. → Correct and more natural; refers to a specific past marriage. 4. Someone married again. → OK, but more natural as “Someone got married again.”

2 is best for the first one. Second one. If I could go back, I wouldn’t have remarried. Or If I could go back, I wouldn’t have married again.

And these; If I could go back, I would never marry him. If I could go back, I would never have married him.

If I could go back, I would never marry (again/anyone). If I could go back, I would have never married. Someone married again. If I could go back, I wouldn't marry ever again. If I could go back, I wouldn't have married ever again.