I read https://english.stackexchange.com/q/103230/50720 and understand the grammatical differences, but their usage still confuses me. Answerer Cereberus writes:

It is slightly old fashioned. Except in negative sentences and questions, I don't think you will see it much in modern writing, except in certain fixed expressions

If I disregard 'old fashioned', then when are the modal and regular 'need' interchangeable, and not? For example, while I understand and thus do not question the Regular uses (indicated by the green checkmarks) here, why are the Modal uses (accompanied by red exmarks) wrong? I don't replicate that Wordreference post because I seek a general answer here.


Modal need is usually restricted to questions and negative sentences. Although people often say that lexical and modal need are interchangable, they have slightly different meanings. Modal need has a deontic flavour. This means that is used to talk about permission and obligation as opposed to pure necessity. Usually modal need implies that some particular person decrees or thinks that it is necessary to do something - or doesn't decree it necessary. This can have all sorts of subtle effects. Consider the following:

  • You don't need to be there till after 5pm.
  • You needn't be there till after 5pm.

The first is quite neutrally saying that it's not necessary for you to be there till after 5pm. The second is saying something like I don't find it necessary for you to be there till after 5pm or I don't oblige you to be there till after 5pm.

I would advise against using modal need in general, because it can have all kinds of subtle nuances that could cause you problems - unless you are very sure of what you're doing.

[Lexical words and function words:

Many grammars talk about function words, the kinds of words you learn about in grammar lessons, and lexical words, normal verbs adjectives nouns and so forth that you learn as vocabulary. Using these categories, modal verbs are considered function words, main verbs are considered lexical words].

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  • Is deontic a grammar term? Grammar A-Z by Oxford has no such term. – rogermue Oct 26 '14 at 5:39
  • @rogermue Yes, it's basic aspect of any discussion about modality in language really. I recommend a grammar such as, A Student's introduction to English Grammar Huddleston and Pullum 2005, or Oxford Modern English Grammar Aarts 2011. If you have access to a good library, the you could try A Comprhensive Grammar of the English Language Quirk et al 1985. There's always A Cambridge Grammar of the English Language H &P 2002. Here'a a page for you: Deontic modality – Araucaria - Not here any more. Oct 26 '14 at 11:11
  • @rogermue See oxforddictionaries.com/definition/english/deontic – Accounting Oct 30 '14 at 9:00
  • Thanks. Would you please explain what you mean by 'lexical need'? We're talking about a word, so by definition of 'lexical', both the regular and need need are necessarily lexical, right? – Accounting Oct 30 '14 at 9:02
  • Will you please to respond in your answer, and not as a comment? – Accounting Oct 30 '14 at 9:03

'Need' as a modal is never followed by a noun.

Do you need some help? XNeed you some help?X

Do you need to go? Need you go?

'Need' as a modal is used almost exclusively in questions and sentences with a negative element.

Need you go? You needn't go.* I don't think you need go.

XYou need goX.

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  • You're answer is generally helpful. However, your comment about being followed by a noun isn't quite correct. If modal need is being used in a question, it will of course usually be followed by a noun - the subject! – Araucaria - Not here any more. Oct 26 '14 at 1:09

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