Whether a word is being used as an adjective or adverb is a question that often arises. Sometimes, this difficulty is associated with the use of prepositions.
A verb, adjective, noun, or pronoun (but not an adverb) uses a preposition to relate to an object, which is either a noun or pronoun:
Go to Beijing (verb relating to noun )
Sit with me (verb relating to pronoun)
Happy with life (adjective relating to noun)
Good for you (adjective relating to pronoun)
Gift from heaven (noun relating to noun)
Cheers to her (noun relating to pronoun)
Something for Xiao Li (pronoun relating to noun)
Everything about this (pronoun relating to pronoun)
I'm next in line: Looking at the preposition "in", the conclusion may be that "next" is an adjective, using "in" to relate to the noun "line". This is not true, because "next" here is an adverb telling more about (modifying) the adverb phrase "in line".
Well, if next were an adjective it would need something to modify; e.g., the next person, and even if it were separated linearly from its noun, it would still have to modify that noun because adjectives can't stand alone:adjective: Sam is happy.
adjective: Sam is sad.
adjective: Sam is next.
Semantics alone tells us that next can't be an adjective; that it isn't nominal--that it doesn't modify a noun, which is the very reason it can't be a subject complement. Semantics.