I understand exactly (please accept that I did not mean to sound smug/arrogant there). After more than forty years of work, I feel that I have a fair understanding of how tense and aspect are used in English, but I still, like Palmer and many others, find modals 'messy' (Modality and the English Modals (Palmer, 1979, p.39)).(It's not exactly the expression I wanted, but I can't describe it, and that's the closest I've been able to find.)I agree. That is one of the things that makes abstract discussion of the words so difficult.I believe intonation and facial expression matter here.
Student or Learner