Onegaishimasu. Years ago at a summer camp I watched this new student always say "Hai" and then just do whatever. It was a little comical, until I noticed that seriousness doesn't hang around long.
I never forgot that wonderful lesson. Even when people couldn't understand a word O Sensei said, they always said "Hai." I hope this little anecdote or two helps.
I can relate to that. I train at Shodokan Honbu dojo in Japan and I only understand maybe 40% of what is said to me, but no matter what I am told, I always say 'hai'...
As for overly talkative sempai, if you don't like it, it's unfortunately an obligatory evil IMHO. It would be very difficult to ask them to shut up in a way that doesn't offend or cause you to miss out on valuable tips in the future.
Everyone has an opinion on how something should be done. Ask 10 people the same question and get 10 different answers. Just nod, say 'ok' or 'hai' or whatever's appropriate and get on with it.
There is someone in my dojo who absolutely always has an opinion on what I'm doing. The tiniest minute details are pointed out and corrected - constantly
, which can be exasperating sometimes when I just want to get on with it and drill whatever I'm doing. But she really knows her onions and nothing she says is redundant. So, as annoying as it sometimes is, I know my aikido is all the better for it, so I put up, shut up and do my best to take on board any tips anyone is good enough to give me. But I do understand your frustration.
...And then there's the supremely rare occasions where I'll do a technique, she'll breakfall, get up, smile and say nothing.