Can is used to express someone's ability
Subject + can + Verb( base form) + Object
Could is also used to request in a polite way
To give permission
To say or express possibility
Subject + may + Verb (simple form) + Object
To get permission
May + Subject + Verb ( simple form) + Object?
To Give permission
Subject + May + Verb (simple form) + Object
Subject + used to + Verb ( base form) + Object
is most commonly used to make recommendations or give advice.
Subject + Should + verb + object
Must is used to express certainly, necessity or strong recommendation. Some times must not is used to express prohibition
Subject + must + verb + Object
is most commonly used to make recommendations.