This is the book _you __ _needed/wanted__the day before yesterday
这是一个定语从句,先行词是book,从句缺宾语,关系代词是that,which 可以省略
知道是定语从句,没补充的时候就已经按照定语从句解释了能再说详细点吗把这个定语从句补充完整，就是This is the book that which_you __ _needed/wanted__the day before yesterday先行词是book,定语从句_you __ _needed/wanted，因为want ,need 是及物动词，所以，从句缺少宾语，用关系代词which ,that,同时，定语从句有个规定，做宾语的冠词代词可以省略