Modal verbs are verbs which modify another verb, and imply the possibility or probability of something happening. Modal verbs are words like can, will, could, must, would, might and should.
After a modal verb, the root form of the word is generally used. The infinitive is not used after a modal verb.
We would to study more of Shakespeare’s works if we had the books.
We would study more of Shakespeare’s works if we had the books.
Exception: The phrase ought to is considered a modal verb. In this case, the to belongs with the ought, and is not considered part of the infinitive.
Pet owners ought to vaccinate all their outdoor pets.
Pet owners ought not let their unvaccinated pets out of doors.
Pet owners ought not to let their unvaccinated pets out of doors.
Modal verbs are always written in the root form, and do not require special endings to identify tense or person.
The president oughted to have approved larger holiday bonuses last year.
The president ought to have approved larger holiday bonuses last year.
We never use more than one modal verb in a clause.
Should he must finish the report by tomorrow?
Should he finish the report by tomorrow?
Must he finish the report by tomorrow?