主语 what my education failed to do；谓语 was；表语 to teach me that the principal significance of such differences was that they were largely without significance
但是表语中还含有表语从句：that they were largely without significance
that the principal significance of such differences was that they were largely without significance做teach的直接宾语（宾语从句）,这个宾语从句中：the principal significance 是主语,was是系动词（谓语）,that they were largely without significance是表语从句.