Very high level students often have gaps in their understanding of the article system in English and sometimes mistakes with articles are the only ones they make. Many English language resources offer very complex rules about the use of articles which can be very difficult to understand and remember. Useful resources do not try to tell you lots of rules which you won’t be able to remember. Two resources to check out:
1. The University of North Carolina. The page here says that an understanding of the article system develops over a long period of time. This development can be stimulated by noticing what you read and referring to the notes on this page as you read. There are some interesting thoughts on how articles are used depending on your meaning and the type of noun. There is also a flow chart which you can follow to help you find which article you should be using. The page is here.
2. The University of Southampton has an excellent resource for raising your awareness of articles. It will take you some time to work through their resource just here.