请不要忘记寄信。

Please don't forget to post the letter.

Please don't forget to mail this letter.

Source: Tatoeba

Sentence Composition

1 请 qǐng
  1. to ask
  2. to invite
  3. please (do sth)
  4. to treat (to a meal etc)
  5. to request
2 不要 bùyào don't!; must not
3 忘记 wàngjì to forget
4 寄信 jìxìn to post a letter; to mail