回信的英文：
reply
write back

参考例句：

I'm in eager anticipation of your reply.
我企盼您的回信。

Threw off a quick response to the letter.
一挥而就地写了封回信

The author whom you criticized in your review has written a letter in reply.
您在评论中批评的那个作者已经写了一封回信.

A return envelope that requires no postage is enclosed for your convenience in reply.
为供您回信方便，随信附去免贴邮票的信封一个。

Punctual in answering letter
按期回信

I had no reply to my letter
我没收到回信

She cannot bring herself to reply.
她不愿回信。

Please write back as soon as possible.
请尽快回信。

I look forward to receiving your reply.
我期待着你的回信。

prepay a letter
预付回信邮费

reply是什么意思：
v. 回应，答复，做出反应
n. 回答，答复

Reply to the sender and all the recipients Reply to the sender and all the recipients Reply All
回复发件人和所有收件人，回复发件人和所有收件人，回复全部

Post reply to the Newsgroup and mail a reply to the author
将回复消息投递到新闻组并向作者回复一个邮件

His challenge draws no reply.
他的挑战没有引出回答。

write是什么意思：
v. 写；写作，作曲；写信

By writing you learn to write
从写作中学写作

legible writing
字迹清楚的书写

Start out to write/with the intention of writing a novel
动笔[想]写一部长篇小说.

back是什么意思：
n. 后面，背面，反面；背，背部；靠背；后卫
v. 支持；后退，倒退；背书；下赌注于
adj. 背部的；后面的；过去的；拖欠的.
adv. 以前；向后地；来回地；上溯

Stand back to back!
背靠背站好！

They are on the back of the menu.
它们在菜单背面。

Our seats on the train were back to back.
我们在火车上的坐位是背靠背的。