Modals of Obligation and Prohibition
When we are talking about obligations we usually use the words have to. This is how we indicate that you have a duty or obligation based on work, family, rules or laws and other factors. Here are a few examples of have to used for obligation:
- I have to pay my bills.
- I had to go to my mother’s house to see her.
- I had to work overtime this week.
- I will have to pick up my brother after school.
Have To Structure
It is common for English learners to think of have to as a modal auxiliary verb. However, it is not a modal verb, or even an auxiliary verb. Keep in mind that have to is a main verb used in the infinitive. It has been added to this lesson to help clear up any confusion. Here is the structure:
subject + auxiliary verb + have + infinitive (with to)
Here are some examples of have to:
|subject||auxiliary verb||main verb have||infinitive (with to)|
|+||He||has||to sing||the song.|
|–||I||do not||have||to visit||today.|
Have To Usage
When you use the phrase have to, usually it is because there is an external power forcing the obligation. This external power could be many things such as the law, school rules, social etiquette, etc.. Here are examples of how to use have to:
- In football, you have to wear a helmet.
- In this game, you have to roll the dice.
- He has to stop at the stop sign.
Did you notice something similar in the examples above? The obligation was never the subject’s opinion or idea. Instead, the obligation is from an outside source. Here are more examples of have to:
|subject||auxiliary verb||main verb have||infinitive|
|past simple||She||had||to cook||last week.|
|present simple||We||have||to study||now.|
|future simple||I||will||have||to drive||next week.|
|present continuous||He||is||having||to wait||until 5pm.|
|present perfect||We||have||had||to walk||to class.|
|modal (will)||They||will||have||to work||today.|
When something is essential or absolutely necessary, we use the term must. This is an obligation that cannot be avoided. Here are a few examples of must:
- I must fix the leak in the roof before the next rain storm.
- She must take her dog to the veterinarian.
- He must stop drinking or he might die.
Must is a modal auxiliary verb and is followed by a main verb. Here is the structure:
subject + must + main verb
Here are a few examples:
|subject||auxiliary must||main verb|
How To Use Must
When we use the word must instead of have to, it is because we are expressing a personal obligation. It is not due to a law, your school or work, as a few examples. The speaker feels that it is necessary for them to do to perform the task.
- He feels that he must get a better job.
- We must sell our house.
- I must fix the car.
In the examples above the obligation is the opinion or idea of the person speaking. It is not an obligation imposed from the outside. It is possible to use must for a real, externally forced obligation, but generally we use have to instead.
- I have to stop working so much.
- I must stop working so much.
In many cases people interchange must and have to. It can be confusing so just remember these other rules for using must and have to:
We can use must to talk about the present or future.
- We must leave now. (present)
- You must go to the park next week. (future)
Must can never be used to talk about the past. If you want to talk about the past, use had to.
- I had to take my mom to the doctor.
- She knew that she had to lose weight.
Must not is used when something is not allowed or permitted. This is used in cases where there are rules or laws in society, which make it similar to uses for have to. The difference is there is not a negative for have to.
- We must not drive over the speed limit.
- She knows that she must not steal candy.
If have to was used for the examples above they would look like these:
- We have to drive under the speed limit.
- She knows that she has to pay for the candy.
Must Not(Mustn’t) Structure
Must is an auxiliary verb that is followed by a main verb. Here is the structure:
subject + must not + main verb
The main verb is the base verb (infinitive without “to”).
|subject||auxiliary must + not||main verb|
Like all auxiliary verbs, must cannot be followed by to. So, we say “You mustn’t arrive late” rather than “You mustn’t to arrive late.”
Must Not Usage
Must not is used when talking about things that are not allowed. They are prohibited and forbidden due to either real laws, regulations or personal obligations. If it is the opinion of the speaker it is considered subjective. If it is due to an external factor such as a rule, law or regulation, it is objective.
Here are a few examples of must not:
- Teachers mustn’t spank children. (objective)
- You mustn’t eat so much sugar. (subjective)
- You must not loiter here. (objective)
- She must not stay up so late. (subjective)
We can use must not to talk about the present or the future:
- You must not drive over the speed limit. (present)
- I must not go there next week. (future)
Must not cannot be used to talk about the past if talking about permission, obligations or similar uses. There are different words to do that. Here are a few examples:
- She could not play in the game.
- I was not allowed to stay out late.
Must not is still used to talk about things in the past but in a different way. It is used to talk about something that did not happen or occur. It is the negative form of must have used to talk about events in the past. Here are a few examples:
- She must not have walked the dog.
- I must not have turned off the lights.