Posted in grammar

Necessity and obligation

It’s time for the next portion of knowledge of deontic modality. Today we will talk necessity and obligation through.

Look at this table:

Need (auxiliary verb)

Need to (full verb)

Need I write this?’ (implicitly: I don’t want to write it)

‘No, you needn’t

‘Do I need to write this?’ (implicitly: I must know)

‘No, you don’t need to write this’

‘I need to leave now’ – assertive context

‘I needn’t have written this’ (but I did it unnecessarily) ‘I didn’t need to write this’ (ambiguity: we know that writing was unnecessary, however, we don’t know if he actually didn’t do that or he did it anyway)

It shows the differences between need as an auxiliary verb and need as a full verb. Although there is a subtle difference in the indicative mood of those verbs, there is a contrast between them in the negative; be aware of it!

Read the examples below to better understand the nature of these verbs and compare them with other possibilities as far as obligation, necessity and the lack of obligation or necessity are concerned:

  • You needn’t / don’t need to be present on our meeting. (Needn’t emphasises the authority of the speaker)
  • ‘Need I take this bag?’ ‘No, you needn’t.’ (Implicitly: I don’t want to take it and I’m waiting for a negative answer)
  • He needs to repair his car. (Assertive context; it is impossible to use need as an auxiliary verb)
  • He need only do it once. (Because only is present, the context is not assertive; that is why need as an auxiliary verb is possible)
  • ‘Do you need to write this again?’ ‘No, I don’t, but I want to.’ (The question about an external need)
  • You will need to / will have to (NOT have to / need to) buy a new shoes if you want to go jogging tommorow. (Necessity will exist if the condition is fullfilled)
  • You need to/have to check the temperature whenever you enter this room. (General/habitual need)
    • Compare: You must check the temperature if you want to enter this room. (I see you want enter this room. I will not be possible untill you check the temperature)
  • I don’t have to go out. (the lack of necessity; NOT I mustn’t go out – it’s a prohibition!)


A language enthusiast, a teacher, a learner and a student of English and French teaching at the University of Warsaw.

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