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:

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:

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.