1 完整句子是 Is this museum _the one （that）_ you visited a few days ago?the one 是先行词 that是关系代词 此处省略
2 my mother and I used to sit in the room in the evening
此处 where = in which
不绝对 where to go 也可以啊 that thing 语法上也没错 所以还是要具体分析 把握几个大的语法点就好了 要会看句子成分