Having trouble explaining why the coordinating conjunction for this sentence is "or", not "and". Please help?

Question

My student's sentence: Getting married before graduating from university is not a shrewd and sensible choice.

I've corrected their choice of coordinating conjunction "and" to "or" [i.e. Getting married before graduating from university is not a shrewd or sensible choice.] but I'm having trouble explaining my instinctual decision. I wonder if it has anything to do with the word "not" and how it might be drawn parallel in usage to the correlative conjunctions "neither... nor"? Or is it because shrewd and sensible are conditions that don't have to be simultaneously applied to the situation?

I would appreciate it if someone could shed some light on this question as I am properly discombobulated!

Answer 1

Very strictly speaking that should be … neither a shrewd nor a sensible choice.

Failing that yes, … not a shrewd or sensible choice.

It’s both about usage parallel to neither... nor and because shrewd and sensible don't have to be applied simultaneously.

Answer 2

May be we can look at this sentence in different forms:

"... is not a [shrewd and sensible] choice."

'Shrewd and sensible' are two adjectives that modify the noun 'choice' at the same time. The choice cannot be shrewd but not sensible or vice versa.

"... is [not a shrewd (choice) ], [nor a sensible choice]."

~ there are two relative clauses

"... is not a [shrewd] or [sensible] choice."

'Shrewd and sensible' are two adjectives that can modify the noun 'choice' independently. The choice can be shrewd but not sensible; or sensible but not shrewd.