1.转为陈述语序：This is the flat in which you once lived.which指代flat,在从句中做in的宾语.
用the one的情况如下：Is this flat the one in which you once lived?转为陈述语序：This flat is the one in which you once lived.从这句话可以看出,is后边缺少表语,所以用the one指代flat,给which引导的定语从句一个先行词.
2.这是一个强调句.Is it in this flat you once lived?判断强调句的方法就是把it is that这三个词去掉,句子仍然是完整的.这句话去掉之后变成：You once lived in this flat.完整的.
原句意为：就是在这个公寓你曾经居住过?