1. The people, whose luggage was lost, had to wait a long time
2. Peter, who couldn't see the screen, decided to change his seat
3. My bike, which I had left in at the gate, had disappeared
Cả 3 câu này đều đã xác định được chủ ngữ nên khi viết lại câu với Mệnh đề quan hệ phải dùng dấu phẩy ","