1. Modal verbs do not take "-s" in the third person.
- He can speak Chinese.
2. You use "not" to make modal verbs negative, even in Simple Present and Simple Past.
- He should not be late.
3. Many modal verbs cannot be used in the past tenses or the future tenses.
- He will can go with us. Not Correct
Can tells about a possibility or a permission. For example:
You can drive if you are over 18. ( Permission)
You can´t vote if you are under 16. (Permission)
You can get bloated if you eat a lot of cheese. (Possibility)