I think it's no use to tell him how to do this.

I found it useless telling him how to do this.

第一句，英语习惯上，在 it's no use 后，一般用 doing ...作外置主语，罕用 to do...。

第一句，英语习惯上，在 useless 后，多用 to do ...，少用 doing...。

所以，应调换过来。

结构：I think it's no use telling him how to do this.（宾语从句）

结构：I found it useless to tell him how to do this. （复合宾语）

区别：think 接宾语从句；find 接复合宾语。第一句，it 是形式主语，is no use 系表，动名词 telling him how to do this 是真正主语。第二句，it 是形式宾语，useless 是宾补，不定式 to tell him how to do this 是真正宾语。翻译基本正确。第二句不是看法，而是过去的一次发现。