这个句子是有Be动词的,完整的句子是：
When they are studying different cultrues and societies,anthropologists often focus on marriage as a contractual agreement between different parties.when引导的时间状语从句中的主语they就是主句中的anthropologists.因为前后主语一致,所以they are可以省略.这是很常见的一种用法.举一个方式状语从句的例子：
He cleared his throat as if to say something.他清了清嗓子,像要说什么似的.
完整的句子是：He cleared his thoat as if he was to say something.前后主语一致的情况下,he was是可以省略的.不会影响句子的理解.解析的前半句就是告诉你这个.
至于解析的后半句是针对这个题目来说的.已经知道有这么一种用法,那么你根据主句的主语要怎么推断从句的谓语是什么样子的.因为是主动关系,所以采用现在分词.举另一个例子来说：
when being questioned,he denied doing anything.这就是主句主语与从句谓语是被动关系的情况.完整的句子是：when he was being questioned,he denied doing anything.
在主从句主语一致的情况下
when done:表示动作完成后发生主句描绘的情形；
when being done:与主句同时发生,主句主语与该动作是被动关系；
when doing:与主句同时发生,主句主语与动词是主动.
如有不懂的 还可以继续问哈