The challenge of learning English is vocabulary. When you're native in one language, and wanted to learn another language you will face a vocabulary problem. You'll not understand every word. In every language, there are many words. They are all different. That's the most challenging of learning another language. There's also another problem is phrases. Native speakers use so many phases. The phases can't be solved by translating the vocabulary. They've different meanings. If we wanted to learn another language we should learn the word and phrases.