We use modal helping verbs to "modify" the meaning of the main verb in some way. A modal helping verb expresses necessity or possibility, and changes the main verb in that sense. These are the modal verbs:

1. can, could - I can't speak Chinese.
2. may, might - John may arrive late.
3. will, would - Would you like a cup of coffee?
4. shall, should - You should see a doctor.
5. must - I really must go now.
6. ought to - We ought to obey the teacher.