"the elderly" = all old people, as a group an elder = a person in a community or a church who is usually older, respected and often has responsibilities to govern or guide the community elder (adj) = older, in this specific contexts e.g. my elder brother elder (adj) = older, respected (like an elder) in some specific contexts e.g. an elder statesman "Elders" is not a synonym for "old people"

They are both nouns. They have the word "THE" in front of them, meaning they are nouns. Elderly can also be an adjective. "He is an elderly man." The elderly is the generic terms for old people. The word elder generally means someone who you should listen to/ respect. "She is your elder." "You should listen to your elders."

Sorry, Dan, but that's not true. "Elderly" is not an adverb. They are both adjectives, and occasionally nouns.

one is an adjective and the other is a adverb