Should must

Modal verbs are used to express certain hypothetical conditions, such as advice, capability, or requests.

Modal verbs show possibility, intent, ability, or necessity. Because they’re a type of auxiliary verb (helper verb), they’re used together with the main verb of the sentence.

They’re used alongside a main verb to change its meaning slightly. Because they’re auxiliary verbs, they can’t necessarily be used on their own. (A modal verb should only appear alone if it’s clear from context what the main verb is.) 

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What if you want to recommend something, but not command it? If you’re giving suggestions or advice without ordering someone around, you can use the modal verb should. 

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​Should shouldn't

​Negative form of should is should not (shouldn't). It means knowing what should not be done. That is, if what one wants to do is not wrong and does not cause any damage but may be offensive and provoke reproof, then one chooses not to do it.

• Should always comes directly before the main verb (except for questions).

• With should, use the infinitive form of the main verb without “to”.

My baby brother should be asleep by now. 

You should try the lasagna. 

If you want to command someone, use the modal verbs must. It is stronger than should.

Modal verbs can express a necessary action, such as an obligation, duty, or requirement. We use must for it.

Must mustn't

The meaning of mustn't is used to indicate that something is forbidden.

We use this to talk about thinks we need to avoid doing.

You mustn't drink sea water.

• Must always comes directly before the main verb (except for questions).

• With must, use the infinitive form of the main verb without “to”.