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That need happen only once for you to be in profit=That needs to happen only once...

Is this need functioning as a modal, just like below, where the bare infinitive is used instead of the to-infinitive due to it being a modal?

He needn't worry=He mustn't worry

Thanks
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Need is indeed a modal auxiliary in your first sentence. It isn't often used in affirmative clauses, however.

He needn't worry doesn't have the same meaning as He mustn't worry. It is closer in meaning to He doesn't have to worry.

CB
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It is closer in meaning to He doesn't have to worry.

Ah yes, thanks. I was a little hasty giving that meaning. I meant what you said Emotion: smile

And yes, the affirmitive doesn't seem to use must in this way often. Clearly it isn't wrong to do so, however.