Research confirms that the findings concerning senior citizens can be applied to restless children. They are more easy-going when there are animals around, with whose company they tend to calm down more easily.

为什么红色部分的 whose 不能用 their，为什么用 their 要在前面加个 and?

【答】网友的意思是：

They are more easy-going when there are animals around, with whose company they tend to calm down more easily. （主从句） 等于：

They are more easy-going when there are animals around, and with their company they tend to calm down more easily. （并列句）

这实际是定语从句和并列句在本质上的区别。前者是含有定语从句的主从句，后者是由and连接的并列句。下面，举例对比讲解：

I met Mr. Smith, with whom I dined at a restaurant.

我遇见史密斯先生，并和他在餐厅一起用餐。

with whom I dined at a restaurant 是定语从句，whom 是关系代词，指代Mr. Smith。

可以改写为：

I met Mr. Smith, and with him I dined at a restaurant.

本句用and 引导了一个并列句，因此改用with him。

又如：

I picked up the apples, some of which are badly bruised.

我拾起了那些苹果，其中一部分已经摔坏了。

也可说：I picked up the apples, and some of them are badly bruised.

The doctor, with whose help the child was saved, is very kind to her patients. 

那位医生对病人很好,就是在她的帮助下,这个孩子才救活过来。

也可说：The doctor is very kind to her patients, and with her help the child was saved.

WHOSE 作为关系代词，之能做定语，用在介词后作宾语的关系代词只有which,whom

(1)介词+关系代词which

I”ll never forget the day on which I joined the League. 

He can't find the pen with which he wrote an English letter.

(2)介词+关系代词whom

I don't know the person about whom you talked just now.

She can't find a person to whom she can turn for a help

The old man has three sons, two of whom are teachers.

The professor of whom a daughter/a daughter of whom has gone abroad is very famous in China.

(3)关系代词whose作定语

The book whose cover is blue is mine.

The student whose father is a doctor is always late for school.

They lives in a room whose window opens to the south. 

I made a table, whose surface is quite smooth.