无论在英语课本还是英语考试中，should 一词的出现频率都很高。should用法灵活，含义丰富，一直是历年英语考试关注的重点，题目也花样百出，让很多同学感到头疼。今天我们来总结一下它的用法，大家分辨清楚。
一 . 作为助动词 shall 的过去式
should 作为助动词 shall 的过去式，可以在间接引语中与第一人称主语搭配，表示过去将来时间。
例如：
The group leader announced that we should(= would)begin to work soon.
小组长宣布：我们不久就开始工作。
A week ago， I told him that I should(= would)go to Beijing the next day.
一个星期以前，我告诉他我第二天就去北京。
二 . 作为情态动词表责任和义务
should 作为情态动词，通常用来表示现在或将来的责任或义务，译作"应该"、"应当"，这时它可以和 ought to， be supposed to 互换使用。
例如：
You should(= are supposed to)complete your test in time.
你们应该按时做完你们的实验。
You should(= ought to)tell your mother about it at once.
你应该立即把此事告诉你妈妈。
In sum， theory should be combined with practice.
总之，理论应该与实验相结合。
三 . 作为情态动词表假设
should 作为情态动词，可以用在条件状语从句中，表示语气较强的假设，译作"万一"、"竟然"，这时也可将 should 置于从句之首，即将 should 放在主语前面，而省略从属连词 if。
例如：
If you should fail to come, ask Mrs. Chen to work in your place. (= Should you fail to come, ask Mrs. Chen to work in your place.)万一你来不了，就叫陈夫人代替你。
If anyone should come， say I am not at home.(= Should anyone come， say I am not at home.)万一有人来访，就说我不在家。
If it should rain tomorrow, I wouldn't go. (= Should it rain tomorrow, I wouldn't go.）
万一明天天下雨，我就不去了。
四 . 作为情态动词表谦逊
should 作为情态动词，可以表示谦逊、客气、委婉之意，译为"可……"、"倒……"。
例如：
I should say that it would be better to try it again.
我倒是认为最好再试一试。
You are mistaken, I should say. 据我看，你搞错了。
He should expect their basketball team to win the match.
他倒是希望他们的篮球队赢得这场比赛的胜利。
Should you like some tea ？ 你可喜欢喝茶？
五 . should 作为情态动词表意外
should 作为情态动词，可以用来表示意外、惊喜或者在说话人看来是不可思议的。尤其在以 why， who， how 等开头的修辞疑问句或某些感叹句中常常译为"竟会"、"居然"。
例如：
How should I know it ？ 我怎么会知道这件事？
Why should you be so late today ？ 你今天怎么来得这么晚？
When I went out, whom should I meet but my old friend Xiao Li ！ 当我出去时，想不到竟会碰见我的老朋友小李！
I don't see any reason why he shouldn't be happy.
我不明白为什么他居然不愉快。
六 . 作为情态动词表推测
should 作为情态动词，可以用来表示有较大可能实现的猜测、推论，通常译为"可能"、"总该……吧"，相当于 be expected to。
例如：
They should be home by now, I think. 我想现在他们总该到家了吧。
The report was written after a careful investigation, so it should be reliable.
这份报告是经过周密调查后才写成的，所以应该是可靠的。
七 .作为情态动词用于状语从句
should 作为情态动词，用在由 so that， for fear that， lest 引导的目的状语从句和 in case(that)引导的条件状语从句中，有"能够"、"可能"、"会"之意。
例如：
They got up early so that they should(= could/ might)catch the first bus in time.
他们起得很早以便能及时赶上头班公交车。
He is working hared for fear that he should fall behind others.
他努力工作，唯恐落在别人后面。
We went over the document again and again lest we should miss any of the main points.
我们把文件看了一遍又一遍，唯恐忽略了其中的什么要点。
He took an umbrella in case（that）it should rain.
他带了一把雨伞，以防天下雨。
八 . 作为情态动词用于虚拟语气
should 作为情态动词，可以用于下列虚拟语气句中：
1. 用在表示与将来事实相反的条件状语从句中，构成" If …… should(do sth）……， …… would/ could/ might（do sth.）…… "句式。
例如：
If it should(或 were to)rain tomorrow, the sports meeting would be put off.(= Should it rain tomorrow/ Were it to rain tomorrow， the sports meeting would be put off.）
如果明天天下雨，运动会就会延期举行。
2. 用在 suggest(propose）， arrange， plan， decide，， advise， order， demand， request， desire， insist 等表示"建议"、"要求"、"命令"、"决定"、"安排"、"计划"、"主张"的动词后面接的宾语从句中。这里的 should 也可以省略。
例如：
He suggested/ proposed/ demanded/ advised that they(should)read the rules carefully.
他建议 / 提议 / 要求 / 劝告他们要仔细阅读这些规则。
The doctor ordered that she(should)stay in bed for another week.
医生嘱咐她再卧床休息一个星期。
He insisted that we(should)take up the matter at the meeting.
他坚持要我们在会上提出这个问题。
3. 用在" It is desired/ suggested/ requested/ ordered/ proposed/ decided "之后由 that 引导的主语从句中， should 也可以省略。
例如：
It is requested that Miss Yang(should)give a performance at the party.
人们要求杨小姐在聚会上表演一个节目。
It has been arranged(planned)that they(should)leave the following week.
已经安排（计划）让他们下星期动身。
It has been decided that the meeting(should)be postponed till next Saturday.
已经决定会议推迟到下周六举行。
4. 用在 suggestion， motion， proposal， order， plan， idea， advice， request， decision， require等名词后面接的表语从句或同位语从句中， should也可以省略。
例如：
We all agreed to his suggestion(proposal)that we(should)go to Nanjing for a visit.
我们都同意他提出的要我们到南京去游览的建议。（ should 用于同位语从句中）
My idea/ motion/ advice/ is that we(should)do more eye exercises every day.
我的意见是我们每天应该多做眼保健操。（ should 用于表语从句中）
5. 用在" It is / was necessary/ important/ strange/ incredible "或" It is a pity/ a shame/ no wonder "之后由 that 引导的主语从句中， should 有"应该"、"必须"、"竟然"、"居然"之意。
例如：
It is necessary that he(should)be sent there at once. 有必要马上派他到那里去。
It is strange that the wheel should turn so slowly.
真奇怪，这个轮子竟然转动得如此之慢。
It is a great pity/ shame/ that he should be so conceited.
真遗憾，他竟会这样自高自大。
九 . " should have + - ed 分词"结构用来表达下述意义：
1. 表示"应该已经……"，"本来应该……"，即过去该做某事但实际没有做，其否定式" shouldn't have + - ed 分词"意为"本来不应该……"。
例如：
These cells should have been kept in some dry places. 这些电池本来应该放（保存）在干燥的地方。（但实际上没有把它们放在干燥的地方。）
You shouldn't have dept those cells in the damp place. 你本来不该把那些电池放在潮湿的地方。（但实际上已经把它们放到潮湿的地方了。）
2. 表示"（估计）应该已经……"，"大概已经……"，即对过去情况的推测。
例如：
They left at nine， so they should have arrived(= probably have arrived)home by now.
他们是九点钟走的，现在应该已经到家了。
They should have finished their work， for they began to do it so early.
他们很早就开始工作，现在大概已经干完了。
3. 意为"竟然已经……"，"居然已经……"，即表示说话人对已经出现的事态感到"惊奇、惊喜、怀疑"。
例如：
I'm surprised that he should have been so foolish. 我很奇怪，他竟然会这么傻。
I'm amazed that Mr. Harris should have said nothing about the matter.
这件事，哈里斯先生竟然只字未提，对此我大为惊异。
It is wonderful that you should have achieved so much these years.
这几年你竟然取得如此大的成就，你真了不起。
此外， should 还可以出现在" should be doing sth "或" should have been doing sth "等句式中，表示"应该正在……"或表示说话人的某种不满情绪。
例如：
Why should we be sitting here doing nothing？ 我们干嘛坐在这里闲着？
You should be wearing a mask. 你应当戴着口罩。
She shouldn't be working like that. She is still so weak.
她不应当那样干。她身体还很虚弱。
You should have been waiting for us. Why haven't you？ 你应当等着我们的。怎么没有等？