The management decided to publicly criticise his irresponsible behaviour to teach him a lesson.

teach 是动词，意思是"教导、教授"。 him 是人称代词 he 的宾格形式，指代前文提到的"他"。 a 是不定冠词，表示"一个"。 lesson 是名词，意思是"课程、教训"。 teach him a lesson 是一个固定短语，字面意思是"给他上一课"，引申为"给他一个教训"，目的是通过批评或惩罚让他记住并改正错误行为。