Modal Verbs for obligation/necessity


1. To help learners to understand the use of modal verbs for obligation/necessity.
2. For learners to be able to choose the right modal verbs to fill in the proper sentence.


1. Study through the content in this chapter.
2. Then, practice and do an exercise after the lesson.

Let’s get to know modal verbs!
Modal verbs function as the auxiliary verbs that mostly used to express the idea of obligation, permission, possibility and or requests.
Modal Verbs position before main verbs which is in a base form; Subject + Modal verbs + Main verbs in a base form.

There are two modal verbs used for obligation:

Must and Have to

Must is for simple present which usually shows general obligations or necessity or strong suggestions like in there examples:
1. You must take the medicine to get better by tomorrow.
2. You must take some rest and go to bed early tonight though.
3. You must see the doctor urgently if you feel worse.

Have to is not an actual modal verb, but it is used like a modal. It is used to express certainly, necessity, and obligation like in these examples:
1. In Thailand, people have to use Thai currency.
2. In France, you have to drive on the right side of the road.
3. In England, most schoolchildren have to ware a uniform.

the difference between 'Must' and 'Have to'

Let's take a look at these two examples first;
- I must stop talking class; otherwise, I will fail this course for sure. (It's my decision to stop talking during class)
- My teacher says that I have to stop talking while she teaching. (I have no choice. It's not my decision to stop talking during class)

We can see that they are used differently depending on who imposes the obligation.
The speaker thinks it is necessary. someone else thinks it is necessary.
For examples:
  1. Imust see doctor tomorrow. (It's my decision)
    I have to see doctor tomorrow.(I don't have a choice)
  2. Imust take 2 pills before having an operation. (It's my decision)
    I have to take 2 pills before having an operation.(I don't have a choice)
  3. Imust go to the hospital this afternoon. (It's my decision)
    I have to go to the hospital this afternoon.(I don't have a choice)
Must not/ Mustn't and Don't have to

Must not/ Mustn't is a negative obligation and shows something that is prohibited while 'Don't have to' shows something that is not necessary like in there examples:
  1. Youmustn't work today. (You can't work today)
    You don't have to work today.(working is not necessary)
  2. Youmustn'ttell the nurse. (It's important not to tell the nurse or don't tell the nurse)
    You don't have to tell the nurse.(You can tell the nurse if you like, but it isn't necessary)
  3. Theymustn't be loud in the hospital. (Being loud is prohibited)
    They don't have to be loud in the hospital.(They can be loud but it is not necessary)
Now that you have learned about obligation with modal verbs, are you ready for some exercise? Now choose the correct verbs for these sentences:

1. You ____ come to this room while they're having an important conference.

2. You ____ buy the train ticket because we can walk there.

3. The rules say that you ____ only invite one guest to the club

4. I ____ stay on for a few hours because I'd rather work late today than over the weekend

5. children ____ ho to school.