What should I do as a TA if the instructor is not teaching properly? Some background: I’m a TA for a low-level course at a major university on the US east coast. This is my second time TAing this course: the first time, I did it under a professor who has taught this a ton of times and knows his way around very well. This time around, the instructor is a second-year grad student who’s teaching anything at all for the first time.

(I’m a second-year graduate student as well. I was actually offered this course to teach this semester, but I said no and so they dropped me in as a TA, which is fine. I have a great deal of sympathy for the current instructor since in another universe that would definitely have been me.)

I’m seeing a noticeable difference between the two. As a TA, I’m getting a lot of feedback from students that they don't understand what’s going on, that the instructor doesn’t relay concepts clearly, that they’re leaving the class more confused than before. The difference is also clear in the homeworks: we’re reusing the old homeworks with minor detail changes, but they’re struggling a lot more in even basic concepts compared to last year. 

My responsibilities in this course are grading, homework fixing, and office hours. I don’t have to teach in this course at all, and I do help those who come in during office hours. Even from my relatively isolated position from the students, though, I’m still getting a ton of pushback regarding the instructor. Should I do more beyond the scope of my duties? If so, what?

Edit regarding those who advised me to tell the instructor: they definitely know about their performance reviews. They pointed out their RMP link to me, and while RMP isn’t the end-all be-all of academic measurement, the feedback there is unusually negative and corresponds with what I’ve been hearing from students. I don’t want to kick the instructor while they’re down, because they already know pretty definitively, and they’re a good person to work with. Just not a good teacher, I guess.
I'd bet that the grad student teaching the course knows very well that his or her teaching competences are lacking, and that the students struggle to follow. After all, even if this person has little experience teaching, he or she has a lot of experience being a student, and is thus likely to recognize poor teaching, but unable to do something about it.

But alas, such is life sometimes. The grad student cannot suddenly, by magic, become a teacher with 20+ years of experience. Nor should they be expected to. The department, who already made the decision of putting an untried grad student in charge of a course, is also unlikely to suddenly allocate more resources.

It would be perfectly fine if you did nothing. Nothing is expected of you, you will not be paid for doing more than you are asked, and if you just do it, no one will thank you.

If you feel compelled to do something anyway, here's a suggestion. Go to the grad student and say that it is your impression that the students struggle way more with the material than last time you TAed the course, and ask if there is anything you can do. Prepare a couple of suggestions, stuff that you know could be improved by going over it a couple of times more in lectures. Don't pass any blame, but try to be as constructive as possible.