Here's a breakdown:
* Before the noun: "The big dog barked loudly." ("Big" modifies "dog")
* After a linking verb: "The dog was happy." ("Happy" describes "dog")
* In a compound noun: "The bright blue sky." ("Bright" modifies "blue sky")
Important Note: There are some exceptions to this rule. For example, some adjectives come after the noun they modify in certain constructions. For example: "Something good happened."