Should is an auxiliary verb - a modal auxiliary verb. We use should mainly to:

  • give advice or make recommendations
  • talk about obligation
  • talk about probability and expectation
  • express the conditional mood
  • replace a subjunctive structure

Structure of should

The basic structure for should is:

subject+auxiliary verb
should		+main verb

Note that:

  • The auxiliary verb should is invariable. There is only one form: should
  • The  main verb is usually in the base form  (He should go).

Look at the basic structure again, with positive, negative and question sentences:

 subjectauxiliary
should		notmain verb
base
+Heshould work.
-Youshouldnotgo.
?Shouldwe help?

Note that the main verb is sometimes in the form:

  • have + past participle (He should have gone.)
  • be + -ing (He should be going.)
The main verb can never be the to-infinitive . We cannot say: He should to go.
There is no short form for should, but we can shorten the negative should not to shouldn't.

Use of should

should  for advice, opinions

We often use should when offering advice or opinions (similar to ought to):

  • You should see the new James Bond movie. It's great!
  • You should try to lose weight.
  • John should get a haircut.
  • He shouldn't smoke. And he should stop drinking too.
  • What should I wear?
  • They should make that illegal.
  • There should be a law against that.
  • People should worry more about global warming.

People often say "They should...do sthg." Usually, the "they" is anonymous and means the government, or the company, or somebody else - but not us! Here are some examples:

  • They should fix this road.
  • They should have more staff in this shop.
  • They should have abolished this tax years ago.

should  for obligation, duty, correctness

Another use of should (also similar to ought to) is to indicate a kind of obligation, duty or correctness, often when criticizing another person:

  • You should be wearing your seat belt. (obligation)
  • I should be at work now. (duty)
  • You shouldn't have said that to her. (correctness)
  • He should have been more careful.
  • Should you be driving so fast?
 

should  for probability, expectation

We use should to indicate that we think something is probable (we expect it to happen):

  • Are you ready? The train should be here soon.
  • $10 is enough. It shouldn't cost more than that.
  • Let's call Mary. She should have finished work by now.
 

should  for conditionals

We sometimes use should (instead of would) for the first person singular and plural (I, we) of some conditionals:

  • If I lost my job I should have no money.
    (If he lost his job he would have no money.)
  • We should be grateful if you could send us your latest catalogue.

This is not a very important distinction. (More about the use of shall/will and should/would.)

should for  If I were you I should...

We often use the conditional structure " If I were you I should..."  to give advice.

  • If I were you, I should complain to the manager.
  • If I were you, I shouldn't worry about it.
  • I shouldn't say anything if I were you.

Note that we can omit "If I were you..." and just say:

  • I should complain to the manager.
  • I shouldn't worry about it.
  • I shouldn't say anything.

In these cases, the phrase "I should" really means something like "you should".

should  for pseudo subjunctive

We often use a special verb form called the subjunctive  when talking about events that somebody wants to happen, hopes will happen or imagines happening, for example:

  • The president insists that the prime minister attend the meeting.

However, this is much more common in American English. British English speakers often convey the same idea using should:

  • The president insists that the prime minister should  attend the meeting.

Here are some more examples:

subjunctive
typically American English		using should
typically British English
The president is insisting that pollution be reduced.The president is insisting that pollution should be reduced.
The manager recommended that Mary join the company.The manager recommended that Mary should join the company.
It is essential that we decide today.It is essential that we should decide today.
It was necessary that everyone arrive on time.It was necessary that everyone should arrive on time.

should for  Why should..?

If we don't understand (or agree with) something, we may use Why should..?, like this:

  • Why should it be illegal to commit suicide? It's your life.

Why should..? and How should..? can also indicate anger or irritation:

  • A:  Help me with this. B:  Why should I?
  • A:  Where are my keys? B:  How should I know?

sources : Original Link