Modal auxiliaries (we are learning: CAN, COULD, MUST, SHOULD) have several special characteristics.
1. They are never used alone. A main verb is either present or implied.
I can fly an aeroplane.
He should behave.
Could you go? Yes, I will (go).
2. Modal auxiliaries have no –s in the third person singular.
I can swim.
She can swim. (NOT She cans …)
I must pass.
He must pass.
They must pass.
3. Modal auxiliaries do not have infinitives (to may, to shall etc.) or participles (maying, shalling, shalled etc.).
You cannot say to shall, to must or to may.
4. They never mix or use an auxiliar or past form.
You can't say: Will Ican? / Do you must? / You musted ...
5. They have "friends" to help them:
CAN/COULD: BE ABLE TOas in `I will be able to go with you´.
MUST/SHOULD: HAVE TO as in `He had to go because his mother told him to´
Still having doubts?
Check the theory in http://www.englishspanishlink.com/verbenglish.htm#modals
and revise CAN /COULD and MUST/ SHOULD / OUGHT TO
Practice with some exercises in http://www.english-test.net/esl/learn/english/grammar/ei007/esl-test.php
