huí xìn
希望早日回信
xī wàng zǎo rì huí xìn
hope to receive a reply soon
给他回了一封信。
gěi tā huí le yī fēng xìn
wrote him/her a reply letter
给哥哥写了一封回信。
gěi gēge xiě le yī fēng huíxìn.
Wrote a reply letter to my older brother.