Most adults who are learning a second language would disagre

Most adults who are learning a second language would disagree with this statement.
这个句子是复合句,可以分解成俩句话
Most adults would disagree with this statement.
Most adults are learning a second language .
即,绝大多数正在学习第二外语的成人不会同意这个观点
Who引导的定语从句限定 adult的范围.并且该句是一个完整的成分主语Who代表Most adults +be 动词+动词ing表示正在

who are learning a second language是定语从句，用来强调adults(成人)。