The professor concluded his lecture by summarizing the main points.

concluded 是动词 conclude 的过去式，conclude 由前缀 con-（表示"完全"）和词根 -clude（来自拉丁语 claudere，意为"关闭"）组成，字面意思是"完全关闭"，引申为"结束、得出结论"。在这里表示"结束讲座"的动作。