I read http://english.stackexchange.com/a/29413/50720 and understand the grammatical differences, but I am still confused by their usage. Answerer Cereberus therein 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](http://forum.wordreference.com/showthread.php?t=2011198&p=10054034#post10054034), why are the Modal uses (accompanied by red exmarks) wrong? I don't replicate that Wordreference post because I seek a general answer here.