7.当主句是以who，which开头的特殊疑问句或先行词是who时，只能使用“that”。
例：Which is the book that he bought
from the library yesterday?(哪本是你们昨天从图书馆借的书？)
注意：●指物时，在介词后只能用which，而不能用that.
●定词从句修饰代表地点、时间的先行词时,要看代表先行词的关系词在定语从句中是否作谓语动词的宾语.如作动词的宾语,就要用关系代词that 或which引导定语从句.反之,要用关系副词 where(表地点)或 when(表时间),在定语从句中作状语.
如:Mary would never forget the
evening when she lost the book. I’ve been to the city
that you visited last week.
被动语态注意事项
●构成
被动语态表明一个被动性动作，它由“助动词be+及物动词的过去分词”构成。
一般现在时 am/is/are + pp.
一般过去时 was /were + pp.
一般将来时 will be + pp. am/is/are
going to be+ pp.
情态动词 may/ can/ must/ should be + pp.
被动语态否定式为be not + pp.，
情态动词被动语态否定式为情态动词+not be + pp..
●主动语态变为被动语态时，通常将主动语态中的宾语变为被动语态的主语；主动语态的主语变为由介词by引导的宾语；谓语动词变为被动语态“be +过去分词”形式，例如：
Everyone likes
the interesting film. (主动语态) → The interesting film is liked by everyone. (被动语态)
They used knives
to cut things. (主动语态) → Knives
are used to cut things. (被动语态)
●某些动词可以跟双宾语如give、pass、bring、write；buy、make等，在变为被动语态时如果直接宾语变为被动语态中的主语，则间接宾语通常由介词to、for引出。例如：
My sister gave me a nice watch for my birthday.→A nice watch was
given to me by my sister for my birthday.我过生日时收到了姐姐的一块手表。
They have bought us many books. → Many books have been bought for us
by them. 很多书都是由他们给我们买的。
●某些动词make、hear、see、watch等在主动语态中，其宾语补足语要求用不带to的不定式。但在变为被动语态时通常省略的不定式符号to必须恢复。如：He made us clean the
classroom after school.→We were made to clean the classroom after school. 我们被迫在放学后打扫教室。
I saw him come into the room. → He was
seen to come into the room. 人们看见他进了屋子。
●在主动语态中如果谓语动词是含有介词或副词的短语动词，变为被动语态时不能丢掉该介词或副词。如：
They must
take good care of the children. → The children must be taken good care of. 孩子们必须小心照看。
●英语中某些动词如sell、lock、draw、smell、touch、feel、sounds、write、be worth doing、sth. needs doing等可以用主动表示被动意义。如：This kind of books sells well. 这种书畅销。 The door won’t lock. 门锁不上。