The man is a doctor. He lives next door. → The man who lives next door is a doctor.
I have a friend. She speaks five languages. → I have a friend who speaks five languages.
Not "The man which lives next door is a doctor." — say "The man who lives next door is a doctor.". 说到人的时候用 who，不用 which。
试一试: I have a friend. She speaks five languages. 把两个句子合成一句。（省略不需要的词。） 答案: I have a friend who speaks five languages. — 朋友是人，所以用 who 连接：I have a friend who speaks five languages. "Which" 是用于事物的，这里不需要。
which 或 that 连接两个关于事物的句子
要补充关于事物或动物的信息时，用 which 或 that 连接，两个都可以。
This is the bus. It goes downtown. → This is the bus that goes downtown.
I like films. They have happy endings. → I like films which have happy endings.
Not "I like movies who have happy endings." — say "I like movies that have happy endings.". 电影是事物，所以用 that 或 which，不用 who。
试一试: This is the bus. It goes downtown. 把两个句子合成一句。 答案: This is the bus that goes downtown. — 公交车是事物，所以用 that（或 which）连接：This is the bus that goes downtown. "Who" 只能用于人，这里不需要。
who / which / that 可以省略
当 who、which 或 that 是承受动作的一方时（也就是别人对它做了某个动作），你可以把它省略："the film that I saw" = "the film I saw"。但如果它是发出动作的一方，就要保留。
This is the film. I saw it last night. → This is the film (that) I saw last night.
Hiroshi is the friend. Mei met him. → Hiroshi is the friend (who) Mei met.
Here is the book. You lent it to me. → Here is the book (that) you lent me.
Not "This is the song I like it." — say "This is the song I like.". 省略连接词的时候，不要再把 it 加回去：要说 the song I like，不要说 "the song I like it"。
试一试: This is the book. Amina recommended it. 把两个句子合成一句，省略连接词。（省略不需要的词。） 答案: This is the book Amina recommended. — Amina 是做动作的人，所以这本书是承受动作的一方——连接词可以省略：This is the book Amina recommended。不需要 "that"，也不能在 recommended 后面把 "it" 加回去，所以两个都不要。
where 连接两个关于地点的句子
要补充关于地点的信息时，用 where 连接。
That's the restaurant. We ate there. → That's the restaurant where we ate.
This is the place. We met here. → This is the place where we met.
Not "This is the place which we met." — say "This is the place where we met.". 说到发生某件事的地点时，用 where，不用 which。
试一试: This is the cafe. We first met here. 把两个句子合成一句。（省略不需要的词。） 答案: This is the cafe where we first met. — 咖啡馆是发生某件事的地点，所以用 where 连接：This is the cafe where we first met. "Who" 是用于人的，这里不需要。
试一试: Amina is the teacher. She helped me. 把两个句子合成一句。选对连接词，省略其他的。 答案: Amina is the teacher who helped me. — 老师是人，所以连接词用 who：Amina is the teacher who helped me. "Which" 用于事物，"where" 用于地点，这里两个都不需要。