how to reply the email from professor?
He gives me an assignment and I want to reply 'I will do it right away'.


Then reply: "I will do it right away".

But if you're going to do it right away, you can probably wait replying until you have finished the assignment, if a reply in that case is expected.


  I ask him some questions about the assignment and he replies something. And I want to express 'yes, got it!'


Then reply: "Got it, thanks."

To sum up: as many students, you're overthinking all this.