(1)必须(used to express obligation)[pt had to](表义务，责任)：I～pay her/keep my word/leave at six.我必须酬谢她/谨守诺言/6点离开。～obey the rules要遵守这些规则;You～take two pills a day/read all your assignments /respect your father's wishes. 你一定要日服两片药/看完全部作业/尊重你父亲的愿望。Visitors～not feed the giraffes/park in front of the entrance/walk on the railway line.禁止游人给长颈鹿投放食物/在门前停放车辆/在铁轨上行走。You mustn't be late again/tell a lie.你不许再迟到/说谎。“M～I go now?”“Yes，you～.”/“No ，you needn't”“我现在就得走吗?”“是的，你必须现在走。”/“不，你不必现在走。”

(2)准是，想必(used to express a probability)(表推测)：He～be ill/exhausted/George.他一定是病了/很累了/乔治。He～be joking/suffering.他一定是在开玩笑/吃到苦头了。We～have taken the wrong road.我们一定是走错了路。It～have been my brother/have stopped raining.那一定是我的哥哥/雨准停了。

(3)硬(偏)要(used to express insistence)(表示坚持)：If you～leave/phone your sister，do it quickly.如果你坚持要走/给你姐姐打电话，就赶快走(打)。I～finish this tonight.我今晚一定要完成此事。Why～it always rain on weekends?为什么老天爷老是在周末下雨?

(4)应该(used to express suggestion or advice)(表建议，劝告)：You～come and visit me/come and have dinner with me sometime.你应该什么时候来看我/与我吃顿晚饭。I～ask you not to say that again.我应该劝你别再说那件事。

【辨异】must与have to、ought(to)和should的区别见HAVE TO。