I have a friend who is Russian and has used German for day-to-day matters for the last two years. He is very well able to have detailed conversations in German. However, his use of articles is often incorrect (unsurprising, since learners of an L2 with articles while their L1 lacks articles often have trouble with this). This is particularly disturbing in written prose.
Basically any kind of imaginable problem occurs:
- Lack of article where one would be expected
- Presence of article where none would be expected
- Mixup of definite/indefinite articles (der/die/das vs. ein)
- Mixup of gender (der vs. die vs. das)
For the last one there are some tendencies but in the end it just requires a lot of practice. The other problems, however, may be different in this aspect.
I have been trying to teach him this indirectly by feigning misunderstanding (e.g. asking for clarification when he should have used a definite article, even though the meaning can be recovered from context). Progress is not invisible but somewhat slow. Because my friend can perfectly use the language he finds it difficult to spend time on doing exercises on this topic, especially also because the exercises you can find are all below his level. What are some other ways to train correct use of articles?