1. where: thay thế từ chỉ nơi chốn, thường thay cho there

   ( where = on/in/at which )

Ex: The apartment wasn't very comfortable. We stayed at that apartment

  → The apartment where we stayed at wasn't very comfortable

  → The apartment at which we stayed wasn't very comfortable

2. when: thay thế từ chỉ thời gian, thường thay cho then

Ex: Do you still remember the day? We first met on that day

  → Do you still remember the day when we first met?

3. why: mở đầu cho mệnh đề quan hệ chỉ lý do, thường thay cho cụm for the/that reason ...

Ex: I don't know the reason. You didn't go to school for that reason

→ I don't know the reason why you didn't go to school