She doesn't often complain, but the fact is she's overwhelmed.

the 是定冠词，用于特指某个事物或情况。 fact 是名词，意思是"事实、实际情况"。 is 是系动词 be 的第三人称单数现在时形式，用于连接主语和表语。 the fact is 是一个固定表达，意思是"实际情况是"，用于引出真实情况或重要信息。