As a Czech native speaker, I did struggle a lot with correct pronunciation. Which of the following parts (or anything else if I have forgotten to mention it) did you find the hardest:
- more complex grammar (inversions, emphatic do, conditionals)
- writing (especially formal and semi-formal texts)
- listening to various dialects and accents (for me, for instance, especially those from the UK)
- the use of English (idioms, word formation, and similar tasks you might encouter in exams)