Just say, "Sensei, I don't think I understand what we're supposed to be doing, can you help?" And then, go with what sensei tells you, and don't expect your partner to be humbled or chastened or feel put in his place or anything, because your practice shouldn't be about him. You are there for your practice, not to improve his character. Get on with your practice and never mind the rest.
I love this approach. Was at a seminar once - still nikyu, struggling to do what I thought (the late) Kawahara shihan had asked us to practice - working with a fellow who was a yondan. The yondan kept correcting me. I asked Sensei because I really didn't get what he was doing. So, I got the explanation in Japanese (which I really don't get very much of), and "wakaru ka".. I understood the movement he was doing (happened to be what I'd been trying to do but didn't have the right ma-ai) but I kept saying um, no (because I didn't understand the verbal part). However, he kept demonstrating on the yondan, which was alright with me, because Kawahara could REALLY cause pain, and I really didn't like this particular yondan that much.