Whays the difference between 입다, 매다 and 끼다?
이는? Is this a typo?
Adding 이 after person's name is to address him/her as 3rd person (beware it's only for persons you're close with).
For 2nd person you add 아 (after consonant) or 야 (after vocal) after their name.
Why is it THE ring and not A ring?
is it wrong to translate this sentences as "Seoyeon wears beautiful rings everyday"?
You can if the context has been established that Seoyeon has a lot of rings that she wears. Typically, you would stress the plurality of "rings" with the plural marker 들.
Does 를 imply that the ring is known to the listener?
Why is it '서연이는' instead of just '서연은' or '서연이'? Wouldn't it be translated to something like 'Seoyeonie' now?
Why is there no plural form of the word "ring"
If you are making a distinction that you are talking about many rings instead of just one you can use the plural marker 들:
서연이는 아름다운 반지를 매일 낍니다. = Seoyeon wears beautiful rings every day.