Childrenbecomeexpertatholdingtheirbreathunderwatereven。。。。。中的expert是什么成分这是新概念二中的一句，原句是：childrenbecomeexpertatholdingtheirbreathunderwaterevenbeforetheycanwalk在我的理解

Question

Children become expert at holding their breath under water even。。。。。 中的expert是什么成分
这是新概念二中的一句，原句是：
children become expert at holding their breath under water even before they can walk
在我的理解中，become是谓语，holding their breath under water even before they can walk是宾语（对吗？），那expert at是什么？是状语吗?可是我看expert是个动词、形容词
有点不解，求高手点拨开窍，感激涕零！

Answer 1

主语是children; become expert at （在某方面成为专家）是一个短语动词，做整个句子的谓语；而holding their breath under water even before they can walk属于动名词或动词ing做宾语。