This is the most interesting book that I have ever read. (4) 当先行词同时包括人和物时，只能用that。如：
Luxun and his works that we are familiar with are
considered great. (5) 当先行词在定语从句中作表语时，只能用that。 如：
■只能用that而不能用which的情形 (1) 当先行词为：all, little, few, much, none 及 some-, any-, no-, every- 与 thing 所组成的复合单 词时，只能用that。如：
There is nothing that will stop us making
why的先行词只能是reason。 关系副词when和where有时可用“介词＋which” 代替，why可用for which代替。
There are occasions _w__h_e_n_(_=_o_n_w__h_ic_h_) one must yield. （屈服） Beijing is the place __w_h_e_re__(=__in_w__h_ic_h_) I was born. Is this the reason _w_h_y_(_=_f_o_r _w_h_ic_h_)he refused our offer?
is very beautiful. （作定语）
■关系副词的一般用法
关系副词有when, where, why，在定语从句中作 状语，分别表示时间、地点和原因。
when的先行词通常是time, day, season, age, occasion等时间名词；
where的先行词通常是place, city, town, village, house, case, situation, scenes等地点或情形名词；
This is the man _w_h_o__helped me yesterday. (作主语)
The teacher (_w_h_o_/w__h_o_m_/_th_a_t__) you want to see
is coming.
(作see的宾语，可以省略) I met a boy w__h_o_s_e_father was a astronaut.
had eaten bad meat.(作状语，用关系副词) 比较：The reason (_t_h_a_t ) he gave for his
absence was obviously fabricated.
(作gave的宾语，用关系代词)
在定语从句中，关系代词 which 和 that 都可指物， 一般情况下，可以互换使用。但在下列情况下值得 注意：
定语从句
the Attributive Clause
1．定语从句：修饰某一名词或代词的从句叫定 语从句。定语从句一般紧跟在它 所修饰的先行词之后。
2．先行词： 被定语从句修饰的词叫先行词。
3．关系词：引导定语从句的词叫关系词。
关系词有关系代词和关系副词。关系代词有that, which, who, whom, whose, as等；关系副词有 when, where, why等。
(2) I’ll never forget the days _w_h_e_n__ we lived together.(作状语) 比较：I’ll never forget the days (__th_a_t__) we spent in Australia. (作及物动词spent的宾语)
(3) The reason (_w_h_y__) she was ill was that she
progress. (2) 当先行词前面有only, some, any, no, every,
little, few, much, all, very等形容词修饰时，只能 用that。如：
This is the only problem that we can’t work out.
(3) 当先行词由形容词的最高级或序数词修饰时， 只能用。如：
关系词通常有下列三个作用： A、引导定语从句；B、代替先行词；C、在定 语从句中担当一个成分。
■关系代词的一般用法
先行词是人，在从句中作主语用who，作宾语用 whom或who，作定语用whose；先行词是物，在 定语从句中作主语或宾语都用which，作定语用of which或whose均可。在限制性定语从句中which, who, whom都可用that代替。关系代词作宾语时常 被省略。
This is a book that I have been looking for since last
week. (6) 当主句是以 which 开头的特殊疑问句时，只能 用 that。如：
Which is the picture that you drew yesterday?
■只能用which而不能用that的情形 (1) 当非限制性定语从句中的先行词指物时，只 能用 which。如：
注意：先行词虽然是时间或地点，但若 在定语从句中作主语或宾语时，要用关 系代词。
(1)The factory _w_h_e_r_e_his father worked has
closed. (作状语) 比较：The factory w__h_ic_h_/_th_a_t__was built in 1978 has closed.(作主语)
(作定语) Here is the coat w__h_ic_h_/_th_a_t_will be made to you.
(作主语）
This is the factory (w__h_ic_h_/t_h_a_t_) we visited last year. (作visited的宾语，可以省略) He has a book w__h_o_se_ cover (=the covero_f_w__h_ic_h_)