<To use any other form of write with a defective auxiliary is always wrong, except for reductions like: He will have. Looks extremely simple to me! If English were an arttificial language, I can't imagine how anyone could possibly make this simpler without adding considerably to the ambiguity of English.>
I see. So when you refer to knowing how to use an item, you mean knowing how to use it syntactically, right? Well, that's probably the easy part of much English usage. Semantic and pragmatic knowledge are often much harder to learn/master.
BTW, what would be the difference between "I know how to use modals" and "I've mastered modals", IYO?
<What is not important are endless arguments about the real or imaginary differences between, say:
He has to go and
He must go.>
Well, I guess that would be part of the mastering part of the language - i.e. the complex part. It's easy for anyone to claim that English is not complex, or is much simpler than many other languages, if he/she avoids talking about mastering the language.
<After the learner has a good command of the basics, he should of course be taught the not-so-common uses that some modals have.>
Again, you seem to be jumping thoughts. Are you saying that the difference in use between "have to" and "must" is not a common thing?
<Compared with other languages, this is not an insurmountable task at all.> *<How surprising that you should know it.>
Again, your view is quite subjective and personal. *Compare the above with other languages by showing us examples from other languages.
<In every language, words have very common uses and less common uses.>
And often, the less common uses are based on differences in register or text type. The language that one needs at any time in his/her learning, and what is or is not common/ frequent for that student, depends on his/her personal situation. For, example, many of my students are lawyers. They need formal language. The above use of "should" would not be uncommon at all in their environment. Therefore, they would need to learn it early in their training.