Modals of permission and obligation

can --> could

verb to be + allowed to --> were/was

may

must (strong obligation) (rules imposed by ourselves)

have to (strong obligation) (rules imposed by authority) --> had to

need to (physical neccesity)

should (light obligation) --> should have + past participle

ought to (light obligation) --> ought to have + past participle

be supposed to (light obligation) --> were/was

mustn't (strong)

can't (strong) --> couldn't

not be allowed to (strong) --> was/were

shouldn't (light) --> shouldn't have + past participle

ought not to (light) --> ought not to have + past participle

not be supposed to (light) --> was/were

don't/ doesn't have to --> didn't

don't/ doesn't need to --> didn't

needn't --> needn't have + past participle