1.Should表示劝告、建议、命令,其同义词是ought to；在疑问句中,通常用should代替ought to.如：You should go to class right away.Should I open the window?Should的含义较多,用法较活,现介绍三种其特殊用法.请看下面的句子：① I should think it would be better to try it again.我倒是认为最好再试一试.② You are mistaken,I should say.依我看,你是搞错了.③ I should advise you not to do that.我倒是劝你别这样做.④ This is something I should have liked to ask you.这是我本来想问你的.从以上例句可以看出：情态动词should用于第一人称时可以表示说话人的一种谦逊、客气、委婉的语气.Should还可以用在if引导的条件从句,表示一件事听起来可能性很小,但也不是完全没有可能.相当于“万一”的意思.从句谓语由should加动词原形构成,主句谓语却不一定用虚拟语气.如：⑤ Ask her to ring me up if you should see her.你万一见到她,请让她给我打个电话.⑥ If you should change your mind,please let us know.万一你改变主意,请通知我们.⑦ Should I (If I should) be free tomorrow I will come.万一我明天有时间,我就来.此外,Why(or How) + should结构表示说话人对某事不能理解,感到意外、惊异等意思.意为“竟会”.如：⑧ Why should you be so late today?你几天怎么来得这么晚?⑨ — Where is Betty living?— 贝蒂住在哪里?— How should I know?— 我怎么会知道呢?⑩ I don't know why you should think that I did it.我真不知道你凭什么认为这件事是我干的.2.“should + have + 过去分词”结构一般表示义务,表示应该做到而实际上没有做到,并包含一种埋怨、责备的口气.如：She should have finished it.I should have helped her,but I never could.You should have started earlier.
