把两句话合成一句话,主要就是找到两句话中同时出现的名词,分清这个名词是指人还是指物I,然后选择相对应的关系词两句话连接起来,主从放在前面,从句放在连词后面；主句中出现过的名词或代词在从句中不能出现.如第一句话中a new school是前后两句话都出现过的指物的名词,所以选which来连接连个句子,而且it在主句中已经出现,所以在从句中就不能出现it了；
I am attending a new school,which is very large.
It has many student,who are from different countries.
We study Maths,English,Pysics and many other subjects,of which i like english best.
An American lady teacher teaches us English,who i admire most.
I also like our History teahcer very much,who is good at telling stories,whcih are very interesting.
I have a good friend whose name is Sally.We often go to the library together,which is a quiet and comfortabe place.
定语从句前必定有一个关系词,这个关系词的作用一是连接主句和从句,而是用来代词先行词的；如果关系词在从句中作主语,那么这个关系词是万万不能省略的,如上面这些句中的who/和一些which是不能省的；但如果关系词在从句中是作宾语的,那么一般情况下就可以省略了,不仅仅是that,who/which用法也是如此.如：
I don't know the man（who/whom/that) you spoke of just now.本句话中关系词在从句中作of的宾语,所以可以省略.当然加上也可以的.希望对你有所帮助.