Personally, as a beginner/early intermediate learner, what I have found works is just to allow my brain to learn this over time by reading and understanding more an more sentences. I used Genki and am aware that it thought this badly. But it was in the 3rd chapter and by the time I reached the 6th chapter I had gotten an intuitive idea anyway (とている just sounded wrong and I couldn't explain why). And I wasn't doing much output anyway so it didn't matter that I maybe confused a couple of verb endings as I recited the 2nd writing exercise og the 5th chapter of my textbook.