你好.你的句子很容易分析.
To find who did this is important
To find who did this（从头到这里是主语） is（系动词做谓语） important（表语）
更容易被接受的是这个句型：
It is important to find out who did this.
1 It (形式主语)is（系动词做谓语） important （表语）to find out who did this.（真正主语）
2 to find out who did this-------这是宾语从句结构,不是宾语从句. find out 是谓语. who did this-----是从句部分.
who是从句部分的主语.did是谓语. this是宾语.
现在,你明白了吗?