that指代人和物
He is a boy who can dance.
this is the book that is interesting.
成分：主语 和定语
在句子缺少主语和定语的时候用that
定语从句中分关系副词和关系代词
when是关系副词
在缺意思而不是缺成分时使用
缺时间意思用when
He bought this toy when he was a 5-year-boy看清楚。。。。时间后