Which is correct to say: "Do you understand" or "Did you understand"?

 It depends on the time frame.

1. If your boss is giving you instructions on a project that he expects you to complete right now, he may ask, “Do you understand?” You may answer, “Sure, boss, I got this. I’ll have this finished by mid-afternoon.”
2. If you screwed up the project yesterday when the boss tried to implemet your work today, he may thunder at you, “Did you understand my instructions yesterday?” You see it all has to do with when the question is asked. It’s a verb tense thing. Hope tjis helps.