What is difficult about English, of course, depends on what your first language is. For instance, I tutor a Chinese woman. She has trouble with he/she/him/her distinction in casual speech since in Mandarin, they both have the same pronunciation (ta1). Also, explaining the pronunciation of words (and how the origin of a word and the history of change in its language effects its pronunciation) is very difficult. I found it helps if you know they know another language and can liken the troublesome segments to that language. For example, the onsets of the final syllables of 'conclusion' and 'retention' correspond to Mandarin 人(ren4) and 車(che1). Last, and probably most importantly, it is extremely difficult (and can really appear to be fairly arbitrary) to explain the how which and why of plurality and mass nouns and the use (or non-use) of articles (a, an, and the). To summarize, native-level English: hard. But with formal linguistic training, its alot better for both you and your students.