1,I lent him the book. I bought it yesterday.
合并后:I lent him the book which ( that ) I bought yesterday. 主句:I lent him the book ,主语 I , 谓语 lent , 间接宾语 him , 直接宾语 the book。
定语从句:which ( that) I bought yesterday, 主语 I, 谓语 bought, 状语 yesterday,which ( that) 关系代词,引起定语从句的关连词,在定语从句中作宾语。