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.

Modal Verbs (II)

Expressing Possibility and Deduction

Possibility is conveyed by using some expressions like perhaps, maybe, possibly, probably, unlikely, improbably, surely and certainly, but also by means of modal verbs such as must, may, could, might and can't.

Possibility can be divided into three categories: certainty, possibility, impossibility. The use of these modals will be ranged according to these three parts, namely must for certainty (100%), may for possibility (50%), could or might for probability (25%) and can't for impossibility (0%).

Look at these examples:
a) Peter must be at the office (= Surely he is at the office).
     Pedro debe de estar en la oficina.
b) Peter may be at the office (= Probably he is at the office).
     Puede que Pedro esté en la oficina.
c) Peter might be at the office (= Unlikely, he is at the office).
     Pudiera ser que Pedro estuviera en la oficina.
d) Peter can't be at the office (= Surely, he is not at the office).
     Pedro no puede estar en la oficina.

In sentence (a) the chance that Peter is at the office is very big, maximum indeed 100%, we are completely sure that this is so. In (b) and (c) there is a possibility that Peter is not at the office, higher in (b) with a 50% and more remote in (c) with a 25% of certainty. And in (d), finally, we are sure that Peter is not at the office (0%), so it is impossible.

Logical deduction can be either positive or negative. Positive deduction is expressed with must and negative deduction with can't.  Observe these two situations:

Everyday my father gets home at 5 p.m.
Now it's five o'clock, and someone is knocking at the door. It must be my father.
Now it's two o'clock, and someone is knocking at the door. It can't be my father.

All this refers to present and future situations. For past situations perfect modals are used, but we will leave them for later, or probably next year.

Practice:
Exercise1, Exercise2, Exercise3, Exercise4.

Modal Verbs (I)

Main Characteristics

Modals are special verbs which behave very irregularly in English. These verbs have common features, which are:

1) They don't need auxiliary verbs for questions and negatives (except 'have to'):
     I can't go to the party.
     Should I give up smoking?
     You don't have to come tomorrow. (exception)

2) They go with a bare infinitive, that is, an infinitive without 'to' (except 'have to' and 'ought to'):
      They could play rugby.
      He might come home next weekend.
      She ought to use a seat belt when she drives. (exception)

3) They don't agree in the 3rd person singular present simple:
     She can play the violin beautifully
     She cans play the flute. (error!)


Expressing Ability and Permission

One possibility of expressing ability and permission is by means of the modal auxiliary verbs can and could, the first one in the present simple and the second one in the past simple and conditional. For other tenses, we need the different forms of be able to for ability and be allowed to for permission. Look at these examples below.

Ability:
When I was eight, I could speak good English.
They can play the piano and the keyboard.
Jennifer will be able to pass the year.
I've never been able to ride a motorbike.

Permission:
We couldn't be late on Saturday nights when we were teenagers.
You can go to the toilet now.
She will be allowed to enter without a ticket.
The children haven't been allowed to play in the gardens.

When asking for permission, leaving aside the differences in use of present and past time, we must also have in mind politeness or courtesy, in other words, formality, where can is less polite than could.
Could I borrow your pen, please?
Can I ask you a question?

May is possible as well when expressing permission, just like could but may is even more formal and polite, and especially used in requests:
May I see your identity card, Sir?
You may leave now if you wish.

Don't forget a thing: when translating can/could, if it expresses permission, just use the forms of 'poder' but if it conveys ability then use the paradigm of 'saber.'

Practice:
Exercise1, Exercise2, Exercise3.