The thought occurred to me this morning that it's John's birthday tomorrow and we haven't got him a present.

网友前面的理解很正确。只有这一种分析，其他分析都是错的。

划线部分为 the thought 的同位语从句。句子主语为 the thought，谓语是occurred to me。同位语从句的位置原本在 the thought 之后，因太长被后置。这种现象叫做“成分分隔”，具体说是“同位语从句被其他成分分隔”。同位语被分隔举例：

There is a new problem involved in the popularity of private cars that road conditions need to be improved.

Information has been put forward at the meeting that more middle school graduates will be admitted into universitie

The question sometimes comes to my mind whether it was worth the effort.

There is no doubt, in my opinion, that what matters is not one’s looks, but one’s inner thoughts

Mr. Smith left word with his assistant that he had returned to his own country for winter holidays.