It's taken for granted that sons and daughters should support their parents.
take...for granted是习惯用语,意思是“认为.是理所当然的”其深层语法结构是复合宾语；
it是形式主语,代替拖后的that主语从句,for granted是介词短语作宾语；
语序可调整为：
That sons and daughters should support their parents (主语从句) is taken (谓语,被动语态) for granted （for granted 宾语）
从句中：
sons and daughters 主语
should support 谓语
their parents 宾语
全句译作：儿女赡养父母被认为是天经地义的.
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