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In this unit you are going to learn how to express obligation and absence of obligation,
permission and prohibition using must, have to, have got to, let, allow, permit, need,
and need to, and how to use should and ought to for suggestions, instructions, duty and
advice.
The principal structures used to say that it is necessary or obligatory to do something
are "must", "to have to", "have got to", "to need to". We'll look at the difference in
meaning between one structure and another shortly.
First let's look at each structure. "Must" is a modal auxiliary verb. It is invariable
in all persons; there is no "s" ending for the third person singular.
I must you must he / she / it must etc. Questions are formed by placing "must" before the subject;
no other auxiliary is used: must I? must you? etc.
The negative is formed by adding "not" after "must". I must not you must not etc.
With the contracted form, "mustn't", the negative is: I mustn't you mustn't he mustn't etc.
Note that we do not pronounce the "t" in "mustn't". The correct pronunciation is /mussnt/. Affirmative
short answers are: yes, I must yes, you must etc.
Negative short answers are: no, I mustn't no, you mustn't etc. "Must" can only be used
in the present tense. It has no past tense and cannot be used with "will" or other auxiliaries.
It has no infinitive or participle forms. To express obligation in any of these cases,
other verbs must be used. "Must" is followed by the infinitive without
"to": I must leave now. You must come and visit us in Paris. You mustn't be late.
"Have" followed by an infinitive is another way to express obligation. In terms of structures,
all the rules for the verb "have" without "to" that were presented in an earlier lesson
apply to "have to": "has" for the third person singular, use of
the auxiliaries "do", "does" in questions, negatives and short answers. "Have to" exists
in all tenses and therefore is used to express obligation when "must" cannot be used:
How I hate having to get up early in the morning! When I was young we had to wear school uniforms.
I guess we'll have to tell them. "Have got" followed by an infinitive is another
way to express obligation. All the rules that were presented earlier for "have got" without
"to" apply to "have got to". "Have got to" is not normally used in the
past and it is without an infinitive and participle forms. We can use the present tense of "have
got to" with future meaning but "will have got to" does not exist:
I've got to go to bed early tonight because I've got to catch an early train tomorrow.
"Need" exists in two forms: as a modal auxiliary and as a normal verb. Right now we are not
going to look at the modal auxiliary as it is only used in questions and in the negative...
... and is never used in affirmative sentences to express obligation. You need carry an identity
card. is a mistake Instead the verb "to need to" which is used
like a normal verb with the "s" ending in the third person singular, and "do", "does"
to form negatives and interrogatives can be used to indicate that it is necessary to do
something. Its meaning is similar to that of "have to",
"have got to" which are used to mean that it is obligatory to do something. Remember
that "to need" is followed by the infinitive: If you go to China, you need to get a visa.
How far in advance do I need to apply?
In the negative there is an important difference in meaning between "mustn't" and "don't /
doesn't have to", "haven't / hasn't got to", "don't / doesn't need to".
First of all, "mustn't" is not used to express the speaker's thoughts or wishes. "Mustn't"
indicates that something cannot be done because it is illegal or highly undesirable: it expresses
prohibition. In meaning, it is similar to a negative imperative:
You mustn't exceed the speed limit. You mustn't eat that. It's spoiled. You mustn't be late.
Remember not to use "have to", "have got to", "need to" in the negative as synonyms for
"mustn't". You can instead use "cannot" / "can't".
You mustn't park here. You cannot park here. or You can't park here.
You can also use the negative forms of the verbs "let", "allow", "permit". They can be
used in all tenses: present, past and future. Used in affirmative sentences these verbs
express permission to do something. The meaning of all three verbs is similar.
In terms of level of formality, "let" is the least formal, "allow" is more formal, and
"permit" is the most formal. "Let" is followed by an object and the infinitive
without "to": My parents let me stay out as late as I like but Tina's parents don't let
her stay out after midnight. "Allow" and "permit" can both be followed
by an object + the infinitive: They allow us to smoke at school but not in class. They
permit students to leave early when they have a request from a parent.
"Allow" and "permit" can also be followed by a gerund: They allow smoking in designated
areas. They permit smoking in designated areas. "Allow" and "permit" can also be used in passive
sentences: Smoking is allowed in the smoking area. Smoking is permitted in the smoking
area. or People are allowed to smoke. People are
permitted to smoke. "Let" is not used in the passive. Use "allow" or "permit" instead.
Used in the affirmative, "let", "allow", "permit" express permission to do something. Used in
the negative they express lack of permission, in other words prohibition.
They don't let us chew gum in class. Chewing gum is not allowed. Students are not allowed
to chew gum. Students are not permitted to leave school early without parental consent.
As you will see shortly when we look at how to express obligation and prohibition in the
past, these structures are extremely useful.
In the affirmative "must", "have to", "have got to" all express the idea of obligation
in ways that are roughly similar. Instead, in the negative, "have to", "have
got to" are similar in meaning to the negative of "need to" and "need" and indicate choice
based on absence of obligation, meaning that you can do something if you want to but that
there is no necessity to do so. As you saw earlier in this lesson, the verb
"to need to" can be used to express the idea that it is necessary to do something.You need
to have a visa to go to China. In the negative, the meaning is that there
is no necessity: You don't need to have a visa to go to the United States.
The modal auxiliary verb "need" is similar in meaning. However "need" is only used in
questions or in the negative, "need not" or the contracted form "needn't".
It is more formal than "need to" and is not used as much. You needn't have a passport
to go to the United States. While it is possible to say that you need
something (or don't need something) using "to need" followed by an object, "needn't"
must be followed by another verb, not by an object:
You don't need a passport to travel in the European Union. is correct You needn't a passport
to travel in Europe. is a mistake The negative forms of "have to", "have got
to" are similar in meaning to the negative forms of "need" and "to need to". They express
the idea that it is possible but not necessary to do something.
"Don't need", "needn't" indicate permission not to do something. "Don't have to", "haven't
got to" indicate absence of obligation. But they do not indicate prohibition; they all
offer a choice. We haven't got to drive so fast. We've got
plenty of time. You don't have to eat the soup if you're not hungry.
We don't have to get up early on Sundays. We can sleep late if we like. There is a vast
difference in meaning between "mustn't", used for prohibition and "don't have to", "haven't
got to" used for absence of obligation. Make sure you understand the difference and
remember to use these verbs correctly. Consider the following: You mustn't drive if you don't
have a licence. You can't drive if you don't have a licence.
You're not allowed to drive if you don't have a licence. You don't have to drive if you
feel tired. I'll drive.
Remember that "must" is not used in the past tense and that "have got to" is rarely used.
Therefore to describe past obligation we use the past of "have to", "had to", using "did"
to form questions and negative sentences. What rules were there when you went to school?
What did you have to do? Girls had to wear skirts. Boys had to wear ties.
Did you have to wear uniforms? We didn't have to wear uniforms but on Mondays, we all had
to wear white shirts or blouses for school assembly.
As you've surely realized, if "must" doesn't have a past tense neither does "mustn't".
It is not possible to use "mustn't" to express prohibition in the past and we cannot use
the negatives of "have to", "have got to" which indicate absence of obligation.
We can use "could not" / "couldn't": In Victorian times women could not vote.
We also use the negatives of "let", "allow", "permit". "Allow" and "permit" are regular
verbs, the irregular past of "let" is "let". Used in the affirmative these verbs express
permission: My parents were very easy-going. They let
me study what I wanted to. They allowed me to choose my friends. We were always permitted
to invite our friends to visit. Used in the negative, in all the ways that
were explained previously in this lesson, these verbs express prohibition: What didn't
they let you do at school? They didn't let us chew gum. Girls weren't
allowed to wear slacks. They didn't allow girls to wear high-heeled shoes, nylon stockings,
make-up or earrings. Wearing short skirts wasn't allowed.
Boys were not allowed to wear jeans. They didn't let boys wear jackets in the building.
Students were not permitted to leave the building during the school day.
"Should" and "ought to" are auxiliary verbs used to show that it is necessary, advisable
or right to do something. "Must" expresses a stronger sense of obligation than "should"
or "ought to". "Should", "ought to" are used for advice,
suggestions or instructions and describing duty rather than for regulations or obligation.
"Should" is invariable in all persons, does not require another auxiliary to form the
negative and interrogative and is followed by the infinitive without "to". The contracted
form of "should not" is "shouldn't". "Ought" is also a modal auxiliary verb. It
is invariable in all persons, forms the negative by adding "not" directly after the verb and
forms the interrogative by placing the verb before the subject. It is the only modal verb
followed by "to". You ought to get more sleep. You ought not
to go to bed so late. What ought they to do? The contracted form of "ought not" is "oughtn't".
Often we use "should" as a more diplomatic way of saying "must", when talking about duty
or obligation. In this case, it would not be correct to use "ought to".
Drivers should stop at pedestrian crossings. Campers should make sure that all fires are
thoroughly extinguished. Library books should be returned on or before the due date.
That is the end of the lesson. Now you can do the exercises on the grammar points presented
in this unit.