1.The illness from which Mary is suffering has now been diagnosed as hepatitis. 這句話的主語是The illness from which Mary is suffering，主語裡面有一個定語從句，which Mary is suffering from，從句裡的主語是Mary， 從句謂語是is suffering from，從句的賓語是which 主句謂語是has been diagnosed as
2.I want to book a room from which I can see the sea.
這個句子的主句是I want to book a room.定語從句是from which I can see the sea.
from which是“介詞+which=關系副詞”這種形式來引導定語從句。例句：
1.The illness from which Mary is suffering has now been diagnosed as hepatitis.
這句話的主語是The illness from which Mary is suffering，主語裡面有一個定語從句，which Mary is suffering from，從句裡的主語是Mary， 從句謂語是is suffering from，從句的賓語是which
主句謂語是has been diagnosed as
2.I want to book a room from which I can see the sea.
這個句子的主句是I want to book a room.定語從句是from which I can see the sea.