I have encountered phrases like these:
La buena comida.
El buen hombre.
Buenos días.
However in none of these instances is it grammatically correct to substitute bien even though it also means good and it is also an adjective (at least as I understand it).
When and how do I use bien as opposed to buen/bueno/buena to describe something good?
The difference is quite simple: bien is an adverb, while bueno/a is an adjective.
You should use bien when it stands alone (esto está bien), when it modifies verbs, specially the participle (esto está bien escrito) or as an intensifier, more or less equivalent with muy (lo tenemos bien difícil, equivalent to lo tenemos muy difícil).
You should use bueno/a when it modifies a noun (era un hombre bueno); if it goes before the noun in masculine singular, bueno becomes buen (era un buen hombre).
After the verb estar, both words are usable, but with different meanings. Unfortunately, the meaning can change between countries. Está bueno in Spain usually means it tastes good, while in some American countries means it is good. Está bien in Spain means it is well done or he feels good, but in some American countries this meaning may change.
While bien can also describe nouns like adjectives do, it's use implies the epitome of good in whatever genre of context it is in.
¡Que bueno! vs ¡Qué bien!
Think of the difference between That's good and F**k yeah, or This place is so great/perfect for us!
Bien is so stupendous that it affects both parties
Sometimes you can hear it being used in place of very or even very well
It can even also be used to describe 'bad'.
Está bien malo eso.
Imagine seeing a scene similiar to those in the Final Destination franchise, those can be described as Very BAD, or rather.... This is horrible!
