Modals

I saw the grammar lessons last week and I have had some problems in understanging those grammar subjects as you might guess that they are modals...

1. ABILITY : can,,could,,,be able to....

in which sentences we can differ them...

2. PERMISSION(asking for) : can,,could,,may,,might,,be allowed to

for which sentences we can differ them.....

3.PERMISSON(giving) : can, may

4. PROHIBITION : mustn't ,,shouldn't


My question that you may not well understandand is that which one we will use ,,ie: under prohibition what is the difference between mustn't and shouldn't .......etc

thanks....