What then when the main predicate is an adjective or a noun?
Jill is busy or Jill is a teacher. We continue to regard the adjective or noun as the predicate of which the subject is the argument, rather than changing and now regarding the copular verb is as the head and busy/teacher as a complement. That is, we produce nsubj(busy, Jill) and nsubj(teacher, Jill). This frequently seems to confuse people, because the main predicate of the clause is now not a verb.