(1)应当，应该(表示责任或义务)(used to express duty or moral obligation)：Every citizen～to help/obey the law.每一个公民都应该提供帮助/遵纪守法。Teachers～to be honoured.教师应该受到尊重。I～not to go on living this way.我不应该再这样生活下去了。

(2)(本)应该(表示劝告或推荐)(used to express advice or recommendation)：You～not to have spoken to her in that way.你本不该那样跟她讲话。You～to have had more consideration for her.你本应该多体谅她。

(3)想必，照说应该(表示揣测)(used to express possibility or natural consequence)：It～to be ready by now.到现在照说应该准备好了。That～to be the postman.那想必是邮递员。If he had started at nine o'clock，he～to have been in London by eleven o'clock.如果他9点出发，到11点时就应该已经到伦敦了。