11.
  The teacher who is always on time was late this morning.

-> (the teacher(chủ ngữ) → who)
12.
 I went to see the doctor who told me to rest for 2 weeks.

-> (the doctor(chủ ngữ) → who)
13.
 It is the first man whom I have loved very much.

-> (the first man(tân ngữ) → whom)
14.
 I often visit my friends in Hue city which is a beautiful city in Vietnam.

-> Hue city (vật) -> which
15.
 I met a famous singer whose mother died last week.

-> (her mother (tính từ sở hữu)→ whose)
16.
 The new stadium which can hold 60,000 people will be opened next week.

-> (the new stadium(vật) → which)
17.
 Nam who is my brother plays the piano well.

-> (Nam(chủ ngữ) → who)
18.
 The woman whom I am talking to is a famous professor.

-> (The woman(tân ngữ) → whom)
19.
 Yesterday I met Nga whom I haven't seen for a long time.

-> (Nga(tân ngữ) → whom)
20.
 The shoes which my mother bought last week are too small for me.

-> (the shoes(vật) → which)