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Have To vs. Must vs. Should 

have tomodalsmustsemi-modalsshould
"Have To" vs. "Must" vs. "Should" in the English grammar

What Is Their Main Difference?

The main difference between 'have to,' 'must,' and 'should' is that 'must' and 'should' are modal verbs while 'have to' is a semi-modal verb.

Semi-modal Have to

'Have to' is a semi-modal verb (also known as semi-modals). Semi-modals function similarly to modals. They sometimes act like modals and sometimes act like a main verb. It is used to express obligations, certainty, etc. For instance:

I have to buy more groceries.

She has to rethink her job choices.

'Must' is a modal verb (also known as modals). Modals give additional information about the main verb of the sentence. 'Must' expresses obligations, likely events, and suggestions. Have a look:

He must be looking for you.

They must come to work this Friday.

'Should' is a modal verb. 'Should' is used to talk about obligations, assumptions, and to give advice. For example:

He should have kept his mouth shut.

She should wake up in a matter of hours.

Similarities

Talking about Obligations

'Have to,' 'must,' and 'should' are used to talk about different levels of obligations:

  • 'Have to' talks about obligations that are self-motivated. Leaving such obligations unfulfilled may not have consequences.

I have to finish my homework by tomorrow.

  • 'Must' talks about obligations that are assigned by law, society, or someone else. Disobeying such obligations will lead to a penalty or punishment.

I must finish my homework.

  • 'Should' expresses obligations mainly when we are criticizing others.

He should have done his homework.

Differences

Negative Form

While the affirmative form of 'have to,' 'must,' and 'should' have the same meaning, their negative forms convey different meanings.

  • The negative form of 'have to,' 'don't have to' is used to say that fulfilling something is not a must and more of a suggestion.

She doesn't have to take out the trash.

  • The negative form of 'must,' 'must not' expresses absolute obligation.

You must not cross the red light.

  • The negative form of 'should,' 'shouldn't' shows that doing something is better to be avoided.

You shouldn't smoke.

Formality and Frequency

The table below illustrates the frequency of 'have to,' 'must,' and 'should' in formal and informal contexts.

Formal Informal
Have to uncommon common
Must common uncommon
Should common common

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