be sure to do sth.和must do sth.的翻译和区别

be sure to do sth一定做某事,是表示说话人的判断,比如:He is sure to come.他一定会来.
must do sth必须/一定做某事,表示说话人的主观打算.如:I must finish my homework.我一定/必须完成作业.