Can
Ability
I can sing
Have to
Express obligation based on external circumstances or rules.
You have to have an email to create a Facebook account.
You don´t have to be so strict about social network rules.
Negative
MUST
Express an emphatic point of view based on the speaker´s opinion
I must have contacts as my friends
You must not be friends with your children on the internet.
Negative
SHOULD
It is used to give advice
You should find your friends from work
You should not your family as friends.
Negative