Modal + infinitive
Incorrect : It may to rain tonight.
Correct : It may rain tonight.
Incorrect : You should to obey your parents.
Correct : You should obey your parents.
Incorrect : You ought obey your parents.
Correct : You ought to obey your parents.
Incorrect : He used visit me regularly.
Correct : He used to visit me regularly.
Note: Most modals are followed by the infinitive of other verbs without ‘to’. Only ‘ought’ and ‘used’ are exceptions. These two modals are followed by a to-infinitive.