Use of Modal Should
'Should' is a modal verb that is used for:
      giving advice, suggestion or recommendation
      predicting the future and talking about expectations
      giving an order or instruction
   1- Giving Advice, Suggestion or Recommendation
'Should' is commonly used to give advice or make suggestions in English. The use of 'should'
in these cases implies a recommendation or suggestion based on what the speaker believes is
the right or appropriate course of action.
Pay attention to the examples below:
When handling food, personal hygiene is essential. Before preparing food, the following
should be done:
      hands should be washed (following the 6 steps)
      clean clothes should be worn
      an apron should be worn
      any jewellery should be removed
      long hair should be tied back, and a net worn.
Advising Not to Do Something
You can advise someone not to do something by using the negative form of 'should', which is
'shouldn't' or 'should not'. For example:
    You shouldn't drink and drive.
    He \should not\ treat his friends like that.
Warning
Do not use phrases like 'I think' or 'I believe' with the negative form of 'should'.
To make such sentences negative, 'think' or 'believe' are used in the negative form, and
'should' is kept as it is. Pay attention to the example:
    I don't think you should eat too many candies.
(NOT I think you shouldn't eat too many candies.)
   2- Predicting the Future and Talking about Expectations
'Should' is also used to talk about a situation that is likely and probable to happen in the
present or the future. It is used to talk about expectations and predictions. For example:
    According to the weather forecast, it should be snowing tomorrow.
    The bus should arrive in ten minutes.
   3- Talking about Expectations in the Past
'Should' can also be used to talk about something that was expected to happen in the past, but
did not happen.
In this case, we use 'should + have + past participle'. For example:
    He should have been more careful.
    It means he wasn't careful enough.
    I should have finished the book by Friday. ( in this example the speaker means 'I didn't
     finish the book by Friday'.)
   4- Talking about Expectations in the Present
'Should' can also be used in the present continuous tense to talk about obligations that are not
being met or somebody who is not acting the right way currently.
In this case, we use 'should + be + verb + -ing'.
    You should be studying for the exam.
    In this case the addressee is not studying.
    We should be helping our mother to make dinner.
   5- Giving an Order or Instruction
'Should' is sometimes used to express an obligation, give orders, or provide instructions.
When used in this way, 'should' is often used to suggest that something is necessary or
expected, rather than to give a direct command. Take a look at the examples:
    All visitors should pay the fee beforehand. (This example refers to an obligation. The
     visitors must pay the fee beforehand. )
NOTE :
Should or Must?
The modal verb 'must' can also be used for giving orders. The difference between 'should' and
'must' is that 'should' is not as strong as 'must'. Using 'should' is more polite.
    All visitors must pay the fee beforehand.
Using ''must'' is more direct.
Should or Ought to
Sometimes 'should' is replaced by the modal verb 'ought to'. 'Ought to' is more formal and it
is not used very commonly in spoken English.
    You should apologize. = You ought to apologize.
'Should + But'
There is a specific grammatical structure in English that involves the use of the modal verb
'should'. This structure typically includes a background sentence, followed by one of the
interrogative pronouns (what or who) placed before 'should', and then the word 'but'
followed by a phrase that expresses a surprising or unexpected outcome.
Take a look at the examples.
    I went to the party and what should they serve but wine.
    I was having fun, and who should I meet but Ian.