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What is the difference between "should" and "must"?

e.g. "They should have called the police." Can I use "must" in that sentence or not?

e.g. "You should read his new book." How about this sentence?

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    I think it's General Reference that in such contexts, should refers to things you ought to do/have done, or that might happen/have happened (but there's always at least a possibility that what should have happened didn't actually happen). On the other hand, must denotes things that cannot be otherwise (if you must do something, you simply don't have the option of not doing it). – FumbleFingers Oct 24 '14 at 17:44
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    I don't think we should close questions about modal auxiliaries as "general reference", as a rule. They're notoriously complex and tricky for learners to master, and no dictionary adequately describes any of them. – snailcar Oct 24 '14 at 17:55
  • @snailboat True, but in that case the question should include more detailed information or research, so that such complexity can be addressed. – user3169 Oct 24 '14 at 20:12
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To answer your first question 'must' is possible in the above example but the meaning will be different than i suppose you have in your mind

When we use 'should' we think what is right/moral/decent thing to do at the time.

When you say 'They should have called the police.' - (Here you know it for sure that police have not been called)

But when you say 'They must have called the police.' (Here you are almost instinctively sure that the police had been called after you have anaylzed the situation well, but officially you don't know whether they called the police or not)

See below how 'should' and 'must' function in your examples

They should have called the police. (It is your opinion)

They must have called the police. (It is your speculation)

For Example

  1. There has been a burglary in one of the houses in your neighborhood. Now, any random family would call the police in this situation. So it is extremely easy for you to speculate that 'They must have called the police' - because that was the sensible and normal thing to do after the burglary.

But when you know that they didn't call the police in this situation then you might express your surprise in the following sentence

  1. I am surprised that they didn't call the police after the burglary. They should have called the police and informed them about it. (Must is not possible in this example)

Another example

Suppse you are telling your friend the following

  1. If you were having such a hard time at school, then you should have told me. (meaning that you might have been able to help your friend out)

Here 'must' is not possible - because you can't compel your friend to share things with you. Sharing is not compulsion. So you can't say --

You must share your problems / feelings with others.

But you say --

You should share your problems / feelings with others.

'Must' in one of its meanings implies compulsion, obligation etc.

To answer your second question -

You should read his new book. (Sounds less convicing than 'must' but it is still a suggestion)

Here 'should' works as a suggestion But You must read his new book. (sounds more convicing)

If there is any book or movie that you have liked very very much then you use 'must' not should. 'Must' will work as 'strongly recommended'

So it should be

You must read his new book. It's amazing.

  • Nice answer, but should have called the police doesn't necessarily mean they didn't. In fact you might well be arguing that they did! :) – Araucaria Oct 26 '14 at 0:33
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Should are Must have different meanings and each with various common usages. In your example, "should have called" means an action did not occur that you would have liked to have occurred.

Also, in your sentence, if you have "must have called", it represents a conclusion to explain a past event.

This usage is not related to a decision of using "should" versus "must".

"I got mugged last week." "Did you call the police? You should have called the police." "I just got mugged." "You must call the police!"

In your examples, "should" represents a suggestion for action, while "must" represents a command where you want the listener to obey you.

Often "should" is used instead of a direct request. "Please pick up your clothes from off of the floor." "You should pick up your clothes from off of the floor." "You must pick up your clothes from off of the floor."

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