1.轉(zhuǎn)為陳述語序：This is the flat in which you once lived.which指代flat,在從句中做in的賓語.
用the one的情況如下：Is this flat the one in which you once lived?轉(zhuǎn)為陳述語序：This flat is the one in which you once lived.從這句話可以看出,is后邊缺少表語,所以用the one指代flat,給which引導(dǎo)的定語從句一個先行詞.
2.這是一個強調(diào)句.Is it in this flat you once lived?判斷強調(diào)句的方法就是把it is that這三個詞去掉,句子仍然是完整的.這句話去掉之后變成：You once lived in this flat.完整的.
原句意為：就是在這個公寓你曾經(jīng)居住過?