You (subject) need
(Verb's Object). ungrammatical
What if auxiliary "need" is used affirmatively and is followed by something that expresses a negative idea or a doubt? It need not be called ungrammatical in such a case.
Affirmative modal forms are possible after negative verbs, and in sentences which express doubt or negative ideas.
I wonder if we need take sleeping-bags.
I don't think he need go just yet.
The only thing you need do is fill in this form.
(You don't need to do anything else)
The only place they need go is the moon. - Whad'ya think a' that one?
He need go no further than the foot of the Cross.
When used as an auxiliary verb, need does not agree with its subject, does not take to before the verb following it, and does not combine with do: He needn't go. Need he go so soon? The auxiliary forms of need are used primarily in present-tense questions, negations, and conditional clauses.
The key word here is "primarily", which doesn't mean "always".