You could have caught the train if you had left five minutes earlier.

could have 是情态动词 could 加动词原形 have 构成的完成式结构，表示"本可以/本能够(做某事)"，用于谈论过去可能发生但实际未发生的事情。这种结构常用来表达遗憾或责备的语气。