Math was the hardest subject for me in school. I always found it difficult because it's so abstract and you need to understand a lot of complex ideas. It was hard to keep up sometimes.
Physics was really tough for me because it has a lot of hard theories and you need to know math well to understand it. It was hard to apply what we learned to real-world situations.
Learning a new language was hard for me. It's tough to learn all the different grammar rules and words, and then you have to know how to pronounce everything correctly too.