Can :
Can poder.
Função:capacity, possibility, ability, request (pedido)
Exemplo: I can drive a car.
May :
May poder.
Função: permission (formal).
Exemplo: May I ask you a question?
Could :
Could poderia.
Função: possibility.
Exemplo: I could buy a new car if I had the money.
Must :
Must dever.
Função: obligation.
Exemplo: You must study hard.
Musn’t :
Musn’t não dever.
Função: prohibition.
Exemplo: You musn’t smoke here.
Should / Ought to :
Should / Ought to deveria.
Função: advice (conselho).
Exemplo: You should take that medicine.