Modal Verbs (III)

Expressing Obligation and Advice

Regarding modal auxiliary verbs, we can express obligation by using either must or have to. Sometimes they both can be used to mean the same thing, but there is a significant difference: generally we use must for expressing personal feelings and we use have to for facts. Therefore, if the obligation is internal and personal then we use must, but if it is an external obligation that is imposed by other people, authorities, rules or laws, then use have to.

Examples:
a) My bedroom walls are horrible. I must paint them as soon as possible.
b) I must brush my teeth twice a day to keep them healthy.
c) You have to turn off your mobile phone in a museum.
d) I have to take an exam to enter that college.

In sentences (a,b) we use must because the obligation here is personal, nobody forces us to paint the walls or clean our teeth. However, in sentences (c,d) have to is used because it is the rules in museums and in that college that oblige us to turn mobiles off and to take entry exams.

Also, must is only used for present and future situations, we use the forms of have to for the past and other verbal forms like the infinitive or past participle. For example:
Today I must study for tomorrow's exam. (present situation)
Tomorrow I must wash my car. (future situation)
Yesterday I had to study for an English exam. (past situation)

Remember that must is just the form of present simple, so we need the forms of have to for the formation of the rest of tenses:
I must go - I had to go - I will have to go - I have had to go, etc.

Finally, don't forget that when the speaker imposes an obligation we use must, and when the speaker does not impose any obligation, but he merely informs an obligation that already exists, we use have to:

Teacher to students:
You must do your homework. (The teacher obliges the students to do it)
Student to mother:
I have to do my homework. (The student just transmits his obligation) 

To express advice we use should or ought to. Both modal verbs mean the same thing, although should does not only express advice but it is also employed as a modal for formal invitation and offering. In this sense, ought to is more formal than should, and at the same time it makes reference to laws and social rules, whereas should rather conveys personal feelings or opinions.  For further information and a more extensive explanation of the usage of these two verbs, just read this article. Must is also used as a modal verb expressing advice in a more emphatic way.

Look at these examples:
You should stop smoking. (a personal piece of advice)
You must stop smoking. (not obligation, but emphatic advice)
You ought to wear a helmet when you ride a bike. (advice concerning the law)

The negative forms of these verbs are shouldn't and oughtn't to.
You oughtn't drink when you drive.
We shouldn't watch that film.


Expressing Prohibition and Lack of Obligation

Prohibition is conveyed by mustn't and lack of obligation by don't have to and needn't. So, don't have to and needn't -which mean exactly the same thing- express the idea that something is not required or necessary, it is however possible if so desired. On the other hand, mustn't expresses the idea that something is forbidden, you cannot do it otherwise you will be punished or something bad will happen to you.

Observe:
You mustn't talk in class. (It's prohibited)
We don't have to come to the office tomorrow. (It's not necessary)
We needn't come tomorrow. (It's not necessary)



Practice:
Exercise1, Exercise2, Exercise3, Exercise4, Exercise5.