I have been speaking and listening to Japanese almost every day for 23 years, and I feel quite confident when I speak it, yet from time to time my wife (who is Japanese) points out quite elementary mistakes in my spoken Japanese. Similarly, most Japanese students make certain elementary mistakes when they speak and write English, even though they have studied it for many years. Three very common examples are:
(a) They forget to use the words 'a' and 'the'.
(b) They do not use plural nouns. For example, they say "I like
strawberry", instead of "I like strawberries".
(c) They say "I was exciting/boring/frightening" etc., instead of "I
was excited/bored/frightened" etc.
In my opinion, Japanese students who want to improve their spoken and written English should concentrate on eliminating common mistakes like this, rather than learning more and more difficult grammar and vocabulary. One needs to know a lot of difficult grammar in order to pass exams, but one does not need to know so much in order to communicate effectively in English.