にラブレターを書いた

wrote a love letter to

→ に
phrase Rare #42,131
wrote a love letter to The に particle marks the recipient. The subject is often omitted when understood from context.
JA: 昨日、彼女にラブレターを書いた。
EN: I wrote a love letter to her yesterday.

トムメアリーにラブレターを書いた

Tom wrote Mary a love letter.

彼女にラブレターを書いたんだよ

I wrote her a love letter.

トムメアリーにラブレターを書いたのに彼女読まなかったんだよ

Tom wrote Mary a love letter, but she didn't read it.