There are several rules for plural forms depending on their endings.
In the examples provided, the one that is spelled incorrectly is "elfs" since the correct spelling for it is "elves".
The spelling rule that applies in this case is that those nouns that end in "f" chage its plural form to "ve". It does not apply to all nouns, but it does in this case. Other example that follow this rule is: "shelf" "shelves",
