There are 24 Auxiliary Verbs in English. They are:
Am Would
Is Shall
Was Should
Are Can
Were Could
Have May
Has Might
Had Must
Do Need
Does Dare
Did Used
Will Ought
The Model "Could" is used to express possibility, permission.
Ex:
1. Ramu could have done better.
2. Could you tell the way to Bus station.
3. Could I talk to Manager.
The Model "Must" is used to express necessity, obligation, duty, determination.
Ex:
I must go to school.
He must catch it.
You must write a letter.
I must complete my home work.
The Model "Could" is used to express possibility, permission.
Ex:
1. Ramu could have done better.
2. Could you tell the way to Bus station.
3. Could I talk to Manager.
The Model "Must" is used to express necessity, obligation, duty, determination.
Ex:
I must go to school.
He must catch it.
You must write a letter.
I must complete my home work.