Meanings
- Ability: can, could; Permission: may, can.
- Obligation: must, have to; Advice: should, ought to.
- Possibility: may, might, could.
Past forms
- could (past ability), had to (past obligation).
Key idea
Modals express ability, obligation, possibility and advice; each modal is followed by a base verb.
Remember
'Must' shows strong obligation; 'should' shows advice.