That【主语】// is【系动词】// why when my PE teacher presented me with the idea of joining the swimming team I decided to give it a shot and challenge myself a bit more【表语从句】// .
备注(1)：why为表语从句连接词。
备注(2)：when为表语从句里面嵌套的“状语从句里”连接词。
备注(3)：绿色字体内容为表语从句的“从句内容”，天蓝色字体内容为表语从句里的“主句内容”
备注(4)：is为主句系动词，presented为why表语从句里嵌套的when引导时间状语从句谓语动词，decided为why表语从句里嵌套的主句谓语动词
备注(5)：通常情况下，谓语动词个数=连词个数+1. 网友给出句子有2个连词，所以谓语动词个数2+1=3. （省略句情况除外）
【1】从句:when my PE teacher presented me with the idea of joining the swimming team 为表语从句里的“从句内容”也就是“时间状语从句”
【2】主句: I decided to give it a shot and challenge myself a bit more为表语从句里的“主句内容”
总结：why引导表语从句里又嵌套“从句+主句”结构，从句为when引导的时间状语从句，主句为“I decided to give it a shot and challenge myself a bit more”
