Can I help you?
An offer for assistance, often from a salesperson. A: "Can I help you?" B: "Yes, I'm looking for a cocktail dress."
Could I help you?
and Can I help you?; May I help you?Could I assist you? (Said by shopkeepers, clerks, food service workers, and people who answer the telephone.) The clerk came over and said, "Could I help you?" Clerk: May I help you? Mary: No thanks, I'm just looking.