Einer von euch ("einer" explanation)
Why do we say "Einer" here with the "er" ending?
Is it because we are referring to the german word "Die Person" and it's dative case here hence "Einer" or?
I believe "einer" is masculine "one" in nominative case... so if you're saying "Einer von euch muss das tun." = "One of you must do that". You are refering to a group of men, or possibly a mixed-gender group.
If you were talking to a group of women, you would say: "Eine von euch muss das tun."
Please someone correct me if I'm wrong :-)
The reason you don't say "Ein" without the "er" ending is that Ein is an indefinite article, and Einer is an indefinite pronoun. If you use an article, you need to put a noun next to it. It could be
Ein Mann von euch - A man of you
The pronoun is used to replace the noun, so you can say
Einer von euch - One of you
both are masucline Nominativ here.
Depending on conext, "Einer" could also be feminine Dativ/Genitiv, for example
Ich muss mit einer Frau sprechen. Mit einer von euch! - With one of you