。
Please don't forget to write the letter.
Literal
Not-forget-[without-ず] letter [object-を] write-[polite-request-てください]
Stripped-down version with the request 書く. The verb choice on 手紙 is itself a small lesson — 書く (write), 出す (mail), 投函 (formally deposit), 投稿 (submit for publication) all operate on letters but belong to different phases and registers, with 投稿 being a clear mismatch for an everyday letter.