needn't/don't need


Senior Member

I'm taking this test:

You may come if you like, but you __________.
A) don't haveC) don't needB) needn'tD) mustn't
I know answer B is correct, but is wrong with answers A and C? Should they have been "don't have to" and "don't need to"?

Thank you in advance.
  • entangledbank

    Senior Member
    English - South-East England
    This is tricky. 'Need' is only a marginal member of the auxiliary verbs. In fact it makes a difference whether it's positive or negative:

    :cross:You need go.
    :tick:You needn't go.

    So, as 'needn't' can be an auxiliary, we can say 'but you needn't'. (You can't say :cross:'But you need'.) As an auxiliary, it is followed directly by the plain verb ('go').

    As an ordinary (non-auxiliary) verb, 'need' and 'have' take a following 'to' before the next verb. This means that in truncated contexts they need the 'to': I need to; I have to. So you can't say 'but I need' or 'but I have' in your sentence - they're not true auxiliaries so you need the 'to' at the end.


    Senior Member
    The meaning of need changes depending on the construction:


    This plant needs watering.
    You need to go.
    I doubt you need to go.

    Modal auxiliary:

    You need not go.

    [You don't have to (or are not required to) go.]

    I doubt you need go.
    [I doubt you will have to (or will be required to) go.]

    Not only need you go, but your whole family as well.
    [Not only do you have to (or are required to) go, but your whole family as well.]

    As far as I know, dare is the only verb that has an infinitive when acting as a modal auxiliary:

    You can come if you like, but I don't dare.

    So in the phrase don't need, the verb need cannot be a real modal auxiliary, so it requires to just as have does.

    It is curious that no to is needed after if you like.
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