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I read http://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.

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.

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, why are the Modal uses (accompanied by red exmarks) wrong? I don't replicate that Wordreference post because I seek a general answer here.

I read http://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.