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.
Best Answer
'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.