sb.是间接宾语，sth.是直接宾语

因为buy sb. sth. = buy sth. for sb.

动词buy可以接双宾语，buy sb sth. 即及物动词+间接宾语+直接宾语。但也可以使用单宾语结构，而将间接宾语用介词for引出。

如:

I am going to buy a gift for her

He gave me a book. = He gave a book to me.

She bought her daught a car. = She bought a car for her daught.

give sb. sth.= give sth to sb.

buy sb. sth. = buy sth. for sb.

1、主语 + 及物动词+ 间宾 + 直宾语 ，间宾在前直宾紧随其后。

2、主语 + 及物动词+ 直宾 + 介词（to/for） + 间宾时，

介词to表示动作方向；介词for表示动作目的

连接间接宾语的及物动词:

用介词to表示动作方向，以to 做介词的有：give pass show tell lend take

用介词for表示动作目的，以for做介词的有：buy cook get sing make

无论是直接宾语，还是间接宾语，都是宾语哦，是谓语动词的【受者】。