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What Is Their Main Difference?
The main difference between modal verbs 'should' and 'must' is that 'should' talks about assumptions while 'must' talks about necessities.
Modal Verb Must
'Must' is also a modal verb. It is used to show necessities. It is also used to talk about likely events and to give suggestions. For example:
We must protect our environment.
You must join the parade.
Similarities
Talking about Possibilities
We use 'should' and 'must' to talk about possibilities and probabilities.
'Should' is used to talk about possibilities with a low chance of occurrence:
She should be at home.
'Must' talks about possibilities that have a great chance of occurrence:
She must be at home.
Negation and Question
We use 'should' and 'must' to create negative sentences and interrogative sentences.
To create negative sentences, we simply add 'not' to the modal verbs as illustrated below:
Should → Should not → Shouldn't
Must → Must not → Mustn't
Take a look at the following examples to see the negation process:
I should attend the class. → I shouldn't attend the class.
He must return the call. → He must not return the call.
We can create yes/no questions and wh-questions with modal verbs:
Yes/no Questions
Yes/no questions are made by putting the modal verbs at the beginning of the sentence followed by the subject and the main verb. For example:
I should do something. → Should I do something?
All pets must be leashed. → Must all pets be leashed?
Interchangeability
Compare the following examples:
You should pay attention to the class.
You must pay attention to the class.
As you can see in the examples above, you cannot use these modal verbs interchangeably. The meaning will be changed significantly when we replace them. In the first example, 'should' lends a sense of suggestion to the sentence, while in the second example, 'must' lends a sense of obligation to the sentence. Here are more examples for clarification:
She should do her homework.
She must do her homework.
Differences
Talking about Necessities
We use 'must' to talk about obligations and necessities. These actions need to be done and are often related to law. We cannot use 'should' in this context since it conveys that the action is better to be done and there is no obligation in it. For instance:
They must clean their wounds.
They should clean their wounds.
Formality
'Should' has an informal nature. 'Must,' however, has a formal nature.
With Conditionals
Conditionals Type 1
'Must' is used with the first type of conditionals. In this type of conditionals, we are talking about real situations and the results that follow them. These situations are considered to have a high chance of occurrence. For instance:
If you attend this class, you must get an A.
If you want this job, you must work hard.
Conditionals Type 2
We can use 'should' in the second type of conditionals that talk about hypothetical situations. These situations are imaginary and are considered to have a low chance of occurrence. For instance:
If you worked harder, you should get a promotion.
If I had that much money, I should travel the globe.
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