B1
Must, have to, should, needn't
Обязанность, необходимость, совет, отсутствие необходимости.
must vs have to
Must — субъективно важно (I must call). Have to — внешнее правило (I have to wear a uniform).
should / ought to
Совет: You should rest. Ought to — чуть формальнее.
needn't / don't have to
You needn't come = не обязательно. Не путать с mustn't (запрет).