Modal Verbs

Modal verbs: ‘Must' and ‘Have to'  

When we want to discuss obligation, that is things that are necessary or important, in the present we use ‘must' and/or ‘have to'. Sometimes the difference in meaning is not always clear but a rule to follow is that ‘must' refers to personal obligation, the speaker has decided that this situation is important to them or it is very necessary. ‘Have to' is used to when the obligation comes from the outside of the speaker.

Subject + must + bare infinitive + object

• He must do more homework to do better in class (To improve, this student has a responsibility to himself to do more homework)
  

• People must feed their children properly (Parents have a responsibility to feed their children)

Subject + have/has to + bare infinitive + object

• She has to wear a uniform at work (Wearing a uniform at work is a strong rule at this workplace)
  

• They have to pay to use the bicycles (You cannot just take the bikes, there are rules about paying from the company that has the bikes)

Negative forms 

The negative forms of ‘must' and ‘have to' are not direct opposites of the positive forms. In the case of ‘mustn't'. We are describing a strong obligation not to do something.

Subject + must not + bare infinitive + object

• You mustn't wear shoes in my house (You have a strong responsibility not to wear shoes here)
  

• People mustn't smoke near gas (It is very dangerous so do not smoke here, it's your responsibility )

With ‘don't have to' we are talking about the fact that there is no obligation to something because often there is a choice.

Subject + don't/doesn't have to + bare infinitive + object

• You don't have to wear a tie in the office it's your decision (The teachers can choose if they want to wear a tie or not)
  

• She doesn't have to pay, I've just paid (‘She' has no responsibility to pay as somebody else has)

Modal verbs: Should and shouldn't  

When we want to describe a weaker obligation, the people involved have more choice or more decisions to make, we use 'should' or 'shouldn't'. Because the obligation contained in these sentences is weaker, we are making recommendations or saying that we think something is or isn't a good idea. ‘Ought to' or ‘Ought not to' are older forms of ‘should' and ‘shouldn't' and often used in very formal situations.

• You should exercise more (It would be a good idea if you did more exercise)
  

• You shouldn't go to bed so late (I recommend going to bed earlier, it's a good idea )r responsibility)

Modal verbs: Question forms  

The modal verbs of strong or weak obligation work conventionally with ‘What/where/when/why/How' question forms or more simple, ‘yes/no' questions

• What do we have to take to the party?
  

• Do we have to pay now or later?
  

• Do you think we should visit him?
  

• Should I open the windows?
  

• What should he do to get better?