1. Modals
English is exceptional in the numbers of auxiliaries it has and the combinations it allows.
Each auxiliary has its own name and position in regard to the others.
Modal auxiliaries express uncertainty (might), necessity (must, should), and possibility (can). Modals do not have agreement or tense endings (hence *he cans; *I am canning to go); they are the first to occur in a sequence of auxiliaries; and do not require an affix on the verb following them (He can walk, but not: *He can walked).
Thus, (7) is a typical instance: the modal could is first and the next verb be does not have an affix: