Modal verbs are special verbs which behave very differently from normal verbs. Here are some important differences:
1. Modal verbs do not take "-s" in the third person.
Julie can speak French.
The team should be here by 6 am tomorrow.
2. You use "not" to make modal verbs negative, even in Simple Present and Simple Past.
We should not eat junk food.
She might not come to college.
3. Many modal verbs cannot be used in the past tenses or the future tenses.
She will can run fast. Not Correct
Naresh musted study very hard. Not Correct
Common Modal Verbs