modals, semi-modals

modals : come from inside, change ( the mode / the power / the possibility / the duty ) of verb.
semi-modals : come from outside

modal (present, future / past)	semi- modal
must ※過去形には使えない	have to
shall (suggestion) / should (these days more formal)	need to (ought to)
may / might	want to
will / would	be going to
can (general ability) / could	be able to (specific event, these days more formal)