REMEMBER
CAN = possible or allowed, colloquial permission asked for. It is usually used in the present.
MAY = probable or polite permission asked for. It is usually used in the present
MUST = sure, logic and advisable.
SHOULD = advisable because it is good but it's up to you.
OUGHT TO = advisable because it is necessary.
TO HAVE TO = imposed by someone or the circumstances. It has got tense form!
COULD = less possible than can or formal permission asked for.
MIGHT = less probable than may
CAN'T = impossible or not allowed by someone.
MAY NOT= not likely to happen.
MUSTN'T = forbidden!
SHOULDN'T = not advisable, but it's up to you.
DON'T HAVE TO/NEEDN'T = it is just not necessary.
