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As a native English speaker, I find the phrase "The hammer need not to be large for..." sounds strange to my ears. Instead, I prefer "The hammer need not be large for...". But what is the rule that makes the former grammatically wrong? Or, maybe I am in error, and indeed the former is technically correct...

marked as duplicate by RegDwigнt Dec 7 '14 at 22:23

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    That's because need is a Semi-Modal auxiliary verb, like dare, and acts like a modal auxiliary (i.e, no to on infinitive complements) in negative environments. Notice that The hammer needs to be large is grammatical, but not *The hammer need be large; that's the way it behaves normally, without a negative around. – John Lawler Dec 3 '14 at 19:14
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    @John: I wouldn't object to The hammer need be large in a literary or poetic context. I would prefer must, though. – Kevin Dec 3 '14 at 19:30

The verb "need" can function as a linking verb or an action verb. As a linking verb, you can say "need + do" just like you say "can + do" but when it functions as an action verb, an infinite will have to follow "need". That is when you say "need to do something".

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