这个很简单啦,which和that引导定语从句时,当在从句中担任宾语成分时,两者是可以省略的,如果指物,则不能用其他引导词代替,指人则可以用whom来代替: This is the mountain village (which/that/不填) I visited last year. 这里,从句中,visit是一个及物动词,后面必须要有宾语,因此宾语从句中缺少的是宾语成分,只有which/that具有这样的功能,而且在担任宾语成分时是可以省略掉的。that引导定语从句在从句中担任主语成分时,如果是人,那么可以用who来代替。
1.限制性定语从句。就是前面有个"," for example: the building,which was built in 1880 , is the highest one in our school. 2.有介词 for example: the students,most of which are under 18,are threntened by the addiction of the internet. 3.who做连接词时,先行词为不定代词,如the one,somebody