Modal verbs don't work the same way main verbs do. Here are some of the grammatical features that make them special:
1. Modal verbs don't add -s to third person singular forms.
It might start at 7 PM.
He can ride a bicycle.
2. In all tenses, even simple present and simple past, you use not to form the negative (rather than do + not)
She would not forget her daughter's birthday.
We could not miss the party!
3. Questions are formed through inversion.
We would read the newspaper everyday if we had a subscription.
Would we read the newspaper everyday if we had a subscription?
4. Typically, modal verbs don't conform to tense rules like other verbs. They don't change to show future or past tense
They had to practice long hours.
Incorrect Example:*They musted practice long hours.*
The students will be able to speak Spanish.
Incorrect example: *The students will can speak Spanish.*
5. Lastly, and most characteristically, modal verbs are followed by the bare infinitive of the main verb - the form without "to."
You could eat dinner earlier.