The role of teaching assistant is new to me, but after one week, I have already learned three things from peeking on the other side of the student-teacher divide.
Realization #1: Teachers have lives outside teaching.
This may be a throwback to the egocentric perspective of childhood, but I am still adjusting to the realization that instructors don't simply appear at the start of class and dissipate after class into the ether to dream up new assignments while us students deal with all the trials of "real life" - all that stuff that we think our teachers don't appreciate, like all the work we have to do for our *other* classes, our mile-long list of errands, that social event that's going to take up our entire Saturday... Turns out, our instructors have stuff like that to juggle, too - stuff that gets in the way of their being able to flawlessly anticipate their students' needs and questions.
Realization #2: Teachers have two jobs.
Like other professionals, teachers have to be knowledgeable about the subject they teach. Unlike other professionals, they cannot dedicate forty hours a week to continually develop that knowledge because they have to - oh yeah - teach classes. In addition to keeping their specialized knowledge current, they also have to develop the skills to effectively convey that knowledge to other people - people of differing backgrounds and priorities who all have their own preferred styles of learning.
Realization #3: Teachers are learning how to teach while they are teaching.
There is no universal solution for getting knowledge into someone else's brain, let alone thirty different brains simultaneously. Teachers have their experience and perspective to guide them, but they also use student feedback to do their jobs effectively.