"Elles sentent bonnes" is incorrect, why can't you change bon is this setting? Thanks :)
In this case, 'bon' is an adverb, not an adjective and doesn't agree with the subject.
You could say for instance -
Ces fleurs ont une bonne odeur.
but it is more French to say -
Elle sentent bon ( ou mauvais) = They smell good ( or bad)
Hope this helps!
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