I should have known he wouldn't come.

known 是动词 know 的过去分词形式。know 的意思是"知道、了解"。 在"should have known"这个结构中，过去分词 known 与情态动词 should 和助动词 have 一起构成完成时态，表示"本应该知道（但实际上当时不知道）"的意思。