I think English Grammar is not as difficult as Vietnamese Grammar. The hardest about English Grammar is the exercises about Sentence Transfromation and Vocabulary Cloze Text in which you must find the words to fill in those gaps in the text. That's really hard and I hope u can show me more exercises about these problems in order for me to practice.
As a native speaker, I know what seems intuitive to me, but looking at it from an outsider's point of view, there are many inconsistencies. Irregular verbs (I am, you are, he/she is); spelling rules (silent letters, "nickEL" but "pickLE", 26 letters but 48 sounds!); subjective vs objective case; active vs passive voice.