Question

difference between must n should

Answer 1

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

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

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

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



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Answer 2

should is used when there is a suggestion or advice, for example: - we should have ice-creams. In this case, there is not any necessity to eat ice-cream. In short, should is used when there is no need of necessity.
must is used when there is necessity for example, we should not eat poisons . In this case, there is necessity, u r strongly suggested to not to eat poisons.
In short, should is used when there is no necessity and must is used when there is a strong necessity.
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