Modal verbs are special verbs which behave very differently from normal verbs. Here are some important differences:
A. Modal verbs do not take "-s" in the third person.
Examples:
1. He can speak Chinese.
2. She should be here by 9:00.

B. You use "not" to make modal verbs negative, even in simple present and simple past.
Examples:
1. He should not be late.
2. They might not come to the party.

C. Many modal verbs cannot be used in the past tenses or the future tenses.
Examples:
1. He will can go with us. ( Not Correct )
2. She musted study very hard. ( Not Correct)