I think forming a sentence in a correct order is the most difficult thing about English. My mother language is Korean and it has totally different word order with English that it takes time in my brain to make correct sentences. For example, in Korean when I want to add a long modifier to a specific word it is usually placed in the front, however in English it is placed at the behind of the word being modified by using relative pronouns such as who or which. I believe that the only way to overcome this difficulty is to be exposed to various kinds of English sentence structures and be accustomed to it, so I'm working on it these days.