Separation anxiety disorder is the child or adolescent's excessive worry and apprehension about being away from their parents. Children with separation anxiety disorder often fear that their parents will be harmed in some way or will not return to them as promised.
fine adjective or adverb. It's an adjective functioning as a Predicative Complement. A Predicative Complement is a Complement of the verb that describes the Subject or Object. In this case it is describing the Subject. Predicative Complements can be noun phrases, adjective phrases and preposition phrases, but hardy ever - if ever at all - adverbs. The reason we use an adjective phrase here is that this phrase is describing a noun phrase. Modifiers of noun phrases are usually adjectives, not adverbs. Consider the following examples: I'm happy. I'm cold. I'm assiduous. *I'm happily. (adverb as PC, ungrammatical) *I'm coldly.
Adjective(noun) not standing by itself; dependent. Adjective(noun) relating to procedure. Adjective(noun) a word used with a noun, or substantive, to express a quality of the thing named, or something attributed to it, or to limit or define it, or to specify or describe a thing, as distinct from something else. Thus, in phrase, a wise ruler, wise is the adjective, expressing a property of ruler.