how to use model auxiliaries
- Modal verbs are NEVER used with other auxiliary verbs such as do, does, did etc.
- The negative is formed simply by adding "not" after the verb.
- Questions are formed by inversion of the verb and subject.
Could you pass me the salt and pepper, please?
- You can never add an "-s" or "-ed" to a modal verb to change its form.
- Except for ought to, modal verbs are NEVER followed by to.
Modal verbs are used to express doubt, certainty, possibility and probability, obligation and permission (or lack of these).
I hope the above information helps.