Question

it is generally said that是什么句型

Answer 1

it是形式主语，指代后置的主语从句。如果用正常语序that is said，其实就是“one says十that引导的宾语从句”的被动语态，未免主语部分太长，显得结构臃肿，所以就改用it is said that句式。
itis saidthat是据说，据传言的意思过去式，it wassaid that +句子，【谓语用过去时】，现在时，it is said that+句子【谓语用现在时】，将来时，it will besaid that+句子【谓语用将来时】。
主语从句，即在复合句中充当主语成分的句子，例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶，此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此“That he finished writing the composition in such a short time”便是该复合句的主语从句。