It's the moment I know that Christmas has truly begun.
老师好!
上面句子我尝试从定语从句(嵌入式定语从句)或强调句等角度去解剖,结果还是理不清语法结构,向老师求教。
It's the moment I know that Christmas has truly begun.
句中的 that Christmas has truly begun 是定语从句,修饰 moment。
句中的 I know 是插入语,它也可移到句首或句末,也可以放在句中并在前后加逗号:
I know It's the moment that Christmas has truly begun.
It's the moment that Christmas has truly begun, I know.
It's the moment, I know, that Christmas has truly begun.