"This is what I should do" 的英文表达是 "This is what I ought to do"。其来源是英语动词 "ought"，表示 "应该，应当" 的意思。例句如下：

I ought to finish this project by the end of the week. 我应该在本周末前完成这个项目。

You ought to apologize to her. 你应该向她道歉。

We ought to leave early tomorrow. 我们明天应该早点离开。