whoever

A fused relative does two jobs at once: it supplies the person and the clause that describes them, so it amounts to "anyone who." The whole clause then acts as a noun. It is the subject in Whoever breaks the law will be punished and the object in fund whoever presents the strongest proposal. Its case is set inside its own clause. That is why the form stays whoever, not whomever, after a verb or preposition when it is the subject of what follows.

One real but unnamed person is meant: someone did rob the old lady, and the speaker simply cannot say who. "The person who" fits in its place but "anyone who" does not, and that test separates it from the free-choice reading. It sits naturally with past events and with requests like ask whoever put it there, where a specific culprit is assumed.

A clause that stands beside the sentence rather than inside it: Whoever stole the painting, the police will catch the thief. Remove it and the main clause is still complete, which is the quickest way to recognize this use. A comma marks the break, and the clause can also trail after the main one (whoever they may be). In the "anyone who" use the same word fills a slot in the sentence instead.

Ever does the work of astonishment here. The word goes back through Middle English whoever to who + ever, and the added word turns a plain question into an incredulous one: Whoever thought up that stupid idea? is less a request for a name than a complaint. In questions like this the one-word spelling is accepted. Outside them, where who and ever simply sit side by side, it counts as a misspelling.

The relative clause has dropped away entirely. Give it to whoever ends with nothing after the pronoun, and the listener supplies "whoever you like." That bare, sentence-final position is what marks it as casual speech. In careful writing, finish the clause (give it to whoever wants it). It suits a shrug like I don't care who gets it, and it can close a list as or whoever.