摘要：如：to whom, for whom, from whom, about whom 等。
"介词"后面通常跟名词或宾格人称代词作为宾语。
whom 是 who 的宾格形式，可作介词的宾语。
如：to whom, for whom, from whom, about whom 等。
who 是主格人称代词，用作主语。
that 是代词，可作任何句子成分。
(一) 可以互换的情况:
A. 作动词的直接宾语时:who 和 whom 可互换，但 whom 在正式文体中更常用。
★You saw who/whom at the party last night?
★Who/Whom did you meet in the street yesterday?
B-1. 指代人时:
介词后面建议使用 whom(正式用法)，that不适用。
who在口语、"非正式文体中"，有时也可用作介词的宾语，但被认为是不正式。
★He is the man to whom I gave the money.
★She is the woman to whom you should address this letter.
这就是你应该把信给与的女士。
B-2. 把介词移到句末时:
此时 who 和 that 可换用，但 whom 不适用。
(原因是who, that...可作主语、引出定语从句，whom不能作主语而引出定语从句)。
★That is the man who/that we were speaking about.
★There is hardly anyone who/that he is afraid of. 我什么人都不怕。
C. 在限定性定语从句中:who 和 that 可互换，但不能用 whom。
(因它是宾格，不能以主语身份引出从句）。
★She is the girl who/that lives next door.
★The person who/that I complained to is the hotel manager.
D. 在非限定性定语从句中:who 和 whom 适用指代人，但 that 不适用。
★The hotel manager, to whom I complained about the service, refunded part of our bill.
The hotel manager, who I complained to about the service, refunded part of our bill.
★My brother, whom you met the other day, has written a book on Chinese art.
E. 在最高级形容词后面:who 和 that 可换用，但以 that 为佳。 (因that具强调意义)
★Newton was one of the greatest men that/who ever lived.
She is the most talented singer that/who I have ever seen.
F. 在 It is/was...句型中:who 和 that 可互换，但 that 更常用,起强调作用。
★It is/was you that/who I wanted to talk to, not John.
我想交谈的人是你，不是John.
★It was she who/that won the first prize in the singing competition.
是她、在歌唱比赛中获得了一等奖。
(二) 不可互换的情况:
A. who 作主语指代人时:
不能用 whom 或 that 替换。
★Who is speaking now? ★Who will come to the meeting?
B. whom作介词的宾语时:不能用 who 替换（在口语中，who 有时会被用作介词的宾语，但这被视为非正式）。
To whom should I address this letter?
★The stewards、cooks etc attending on the passengers are not the part of crew for whom spaces are to be deducted. 摘自《巴拿马运河丈量规则》
伺候乘客的服务员、厨师等不属于被扣除居住空间的船员部分。
The building was picketed by demonstrators, several of whom were well-known actors.
这座大楼遭到示威者的围攻，其中有几个是的知名演员。
★I learned the news from whom I had expected to hear nothing.
我从预料不会听到任何消息的人那里得到了消息。 ★That is the man about whom we were speaking.
C. 固定说法/谚语中不能互换:
★Whom the gods love die young. 好人不长寿 。
总结:
通过上述例句，我们得知: 1/ 作动词的直接宾语时：who和whom在正式文体中可以互换，但whom更常用。that不能用于介词的宾语。
2/ 介词后面：
建议使用whom（正式用法）。 who在口语或非正式文体中有时也可用，但不正式。that不适用。
3/ 介词移到句末时：
who和that可换用，whom不适用。
4/ 限定性定语从句中：who和that可互换。 whom不适用。
5/ 非限定性定语从句中：who和whom适用指代人。that不适用。
6/ 最高级形容词后面：who和that可换用，但that为佳。
7/ It is/was...句型中：
who和that可互换，但that更常用。
8/ 不可互换的情况：who…作主语指代人时，不能用whom或that替换。
whom…作介词的宾语时，不能用who替换（口语中非正式用法除外）。
来源：鲲鹏展翅一点号
