Is this museum the one they visited last month?为什么是the one而不

先把句子换成陈述语得：
This museum is the one they visited last month.
如果换成用that就成为：
This museum is that they visited last month. 很显然that引导的就是一个表语从句,但它不能在从句中充当句子成份,也没有实际意义,只起连接作用,一般不能省略.可是,在从句中visited是及物动词,后必须带宾语意思才完整,所以换成that从句中就缺少应有的宾语,出现句子成份残缺,也就是错误的表达.正因如此,就只能用the one用来代替前文中所说的同类事物中的其中之一.
同样： Is this the museum_____they visited last month?
A that B where C which D the one
换成陈述语序就在为：
This is the museum ___ they visited last month. 此时就要用that或which,但它们在定从中作宾语,也可以省略.
如还不明白,欢迎追问.