This is the 875th most frequent Korean word.
돈이
"돈이" means "money" (subject marker attached).
돈이 is the subject of the sentence, indicating possession of money.
돈이 많으면 기분이 좋아요.
If you have a lot of money, you feel good.
돈이 is used as the subject suggest needing money.
돈이 필요하면 알려주세요.
If you need money, please let me know.
돈이 is the subject here, highlighting the lack of money.