Zhang Zhaoyu is a really very laid back professor, but he is not the best lecturer. When he lectures he usually just annotates the lecture slides and goes through the information pretty fast. It was difficult for me to honestly retain anything he was lecturing about. My grade is mostly because I self-studied. If you do happen to take this class, I recommend checking out Organic Chemistry Tutor. OGT honestly saved me, as you can find all 31b concepts on the channel explained very clearly. Zhang's tests were pretty fair. As long as you are able to do the homework, you should be able to do well on Zhang's tests. The tests are also graded very leniently.

Prof Zhang is a great person. He is kind and funny. However, his accent makes things hard to understand, and he has a very quiet, introverted voice for a class that has 200 students in it. While my grade is mostly due to my lack of effort in studying the material, the effort was lost due to his extremely unengaging class. I felt no merit or reason to go to his lectures because I felt like I did not learn anything. The youtube math channels helped me much more, and only if I studied harder and earlier for the final, I would have ended with a much better grade. That said, he is a horrible instructor, and I would strongly recommend you choose another option if available or take it another semester. You should only take this class with him only if all of the conditions below are met:
1. You need to take Math 31B this quarter
2-1. Your only option or time available is this class
2-2. your other option is Ocal
3. You are willing to self-study via youtube

If these conditions are met, it is ok to take this course since you have no other choice.

That said, I publically condemn the UCLA Math Dept. for purposely putting the worst math instructors at UCLA for this significantly important class for many majors. Also, I originally had Li, but they just swapped him with Ocal, and then swapped again with Zhang. They should do this before students start course registration since the instructor is one of the biggest factors in choosing a class.