English and Japanese are very different in grammar and syntax. When I face a difficulty, I usually consult LDOCE. When I am writing in English, good dictionaries and well-written grammar books are essential. But I don`t think any language can be studied alone because the gaps between different languages are very tricky.
I was in England alone when I was 40 years old. I had never gone abroad before; I am not, and I had not been good at speaking English. After a week or so, I felt intense loneliness, experiencing a kind of culture shock.
I visited a bookshop near St. Paul's in London, and bought two Japanese magazines and a Japanese newspaper. As soon as I began reading those, I was able to recover from the disorientation of my mind. At that time I felt that I should have carried small books written in my native tongue. After a few weeks I was able to enjoy my stay there, although my English did not improve very much. That was 20 years ago.