${1.}$ I wrote an email to my sister who lives in Italy.
→ dùng MĐQH "who" thay thế cho "my sister" ( tức là "she" vế sau ) - N(người) + who/that + V
${2.}$ The man who is wearing a blue hat is in the class.
dùng MĐQH "who" thay thế cho "the man" ( tức là "he" vế sau )
${3.}$ The phone which/that belongs to Ann is on the table.
→ có thể dùng MĐQH "which/that" thay thế cho từ " the phone " ( tức là "it" vế sau ) - N(vật) + which/that
${4.}$ This is the laptop which my mother has just bought.
→ dùng MĐQH "which" thay thế cho từ " the laptop " ( tức là "it" vế sau )
${5.}$ I know the woman who lives upstairs.
→ dùng MĐQH "who" thay thế cho "the woman" ( tức là "she" vế sau )