Must and have to are particularly important to express: obligation, necessity, prohibition (in negative), positive deduction, etc. The correct use of them is not very easy...
- Must and have to are used in affirmative to express obligation; must is often associated with an inner feeling of duty or personal or specific obligation on the part of the speaker (I must go now) whereas have to represents "the law", it usually refers to general rules, external obligations or impositions (You have to wear the helmet). Apart from this different connotation, must and have to are quite interchangeable in the present and future. In the past only have to is used.
- In the negative mustn't is used to express prohibition whereas have to refers to lack of obligation, something quite different.
- Should and ought to are mainly used give advice or refer to "moral" obligation or manners.
Our friend Ana Flores has sent us a pps showing the use of these modals. Have a look at it, it's very clear.
Also have a go at this video explaining the use and conjugation of have to and must.
