The modal verbs should, ought to, have to and must are all used to show obligation. The tone and strength of the obligation can vary based upon which modal is being used.

Should vs Ought to (for mild obligation / strong advice)

Look at the following sentences:

You should go to hospital with that wound.
You ought to go to hospital with that wound.

In these two sentences the function of should and ought to are interchangeable. Both focus on a strong advisability, or in other words a mild obligation. You are obligated to care of yourself. Nowadays, the use of ought to has lessened and should is commonly used in its place.

Have to vs Must (for strong obligation / necessity)

Here are the same sentences using the modal verbs have to and must:

You have to go to hospital with that wound.
You must go to hospital with that wound.

Have to and must are considered stronger than should and ought to. Both modals carry the function of necessity, obligation or even advice, but mustHave to, is normally reserved for expressions related to the law. For example: If you own a car, you have to pay an annual road tax. On the other hand, must is normally reserved for giving orders that people are obligated to follow. Here are some further examples of more typical usage of have to and must: is considered the strongest modal.

You have to pay income tax.
You have to pass your driving test before you can drive alone.
You have to show your passport when you pass through immigration.

You must get to work by 9am.
You must get this report finished by 30 June.
You must attend the meeting.

 
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