Hi. I was wondering about something my teacher corrected in a test exam I had; the expression "need not be." I wrote: "This need not be", and it was corrected to "This needs not be". I was just curious whether this is correct, because I have seen many examples of both versions. I thought that in this context "need" was some sort of a subjunctive word.

Not subjunctive, but a modal auxiliary. 'Need' can be used as either:

Modal auxiliary -- 'He need not worry about anything during his stay on Paradise Island.'
Main verb -- 'He does not need to worry about anything during his stay on Paradise Island.'

They carry the same meaning. Your test answer is correct (but arguing with a teacher offers certain dangers).
Google massively favours 'this need not be'. Maybe that would convince the teacher in question.

Students: Are you brave enough to let our tutors analyse your pronunciation?
Just to be sure:
Need as a modal auxiliary is only possible in its negation, am I right? Or are there any examples for "need" used as an auxiliary in non-negated sentences?
There must be a negative word or implication in the clause, K., when 'need' is a 'semi-modal auxiliary':

You needn't go.
Only he need go. [i.e. no one else]
Need I go? [i.e. 'probably not' or 'I don't want to go']

Modal "need" must be non-assertive, i.e., in negations or in questions.
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Thanks for all the answers, guys. In case I would need some more "hands on" proof, I went to the library and looked it up in the BBC English Dictionary (a BIG book) and found the following:

"2. SEMI-MODAL If you say that something NEED NOT happen, or that someone NEED NOT do something, you are saying that there is no good reason for it to happen."

If that doesn't convince the ol' geezer, I don't know what will.

Thanks again!