Don't forget to stick a stamp on an envelope before you put it into the mail-box.

把信投进信箱前别忘了在信封上贴上邮票。

Don't forget to stick a stamp on the envelope...

不要忘记在信封上贴邮票。

Don't forget to stick a stamp on the envelope.

不要忘记在信封上贴邮票。