DON'T HAVE TO
SHOULDN'T
SHOULD
NEED TO
HAVE TO
MUST
DON'T NEED TO
MUSTN'T
Suggestion. Not an obligation, but it is best to do as adviced.
Suggestion. Not a prohibition, but it is better not to do it.
Obligation, strong.
Optional, not necessary but not wrong to do or have.
Obligation, not as strong.
Optional, absolutely no obligation or phohibition of any type.
Prohibition, strong.
Necessity. Strong.