The most difficult courses have their own opinions in many other disciplines, but in the department of mathematics, the opinions are often unified: real variable function, functional analysis and topology, abstract algebra (modern algebra). Many students also think these courses are difficult. It should be said that the department of mathematics is upgraded on the basis of studying mathematical analysis and advanced algebra (basic course), which is regarded as an advanced course. Whether it is suitable for learning mathematics depends on your mastery of mathematical analysis.
I agree with the small point that mathematical English is very important, because the English translation of many nouns in mathematics is very strange, for example, power means power.