当你看到这封信的时候，请给我回信好吗！用英语怎么说！！

当你看到这封信的时候，请给我回信好吗
when you receive this letter, please reply to this letter, would you?

首先我们都知道，letter is to be read not be seen.英语的习惯是说成读信而不是看信。see的用法就是不对的。

即使你想表达让对方把信看完，再给你写回信，你也要至少这样表达，When you read the letter, please reply to this letter.

回信英语的说法是reply to a letter，非常明确的地道的表示回信的一种表达。

请please好吗？would you? 或者 will you?其中would you比较委婉一些，如果你不是特别在乎对方的反应的话，一般用will you?来表达也是可以的。

还有一种更好的表达方法：
Would you reply with a note on receiving this letter
Would you reply with a note upon receiving this letter

on receiving, upon receiving
在收到...的时候，在收到之后...的时候

When you see this letter,please sent a letter back to me!

这里不能用return，让人感觉你是要让他把原信还给你似的，应该用write back；后面用的词不管写什么对方都能看得懂，无所谓

When you see the letter, please give me the reply.

When you see this letter, please give me a reply!

would you please write back when you see this letter?