The speaker illustrated his thesis with three detailed case studies.

illustrated 是动词 illustrate 的过去式。illustrate 由前缀 il-（表示"进入"）和词根 lustr（"光亮"）加上动词后缀 -ate 组成，原意是"照亮、阐明"，引申为"用例子说明、举例证明"。在句中表示"通过具体案例来展示和证明"。