我会想你的。
I'll miss you. *miss “想，想念”。
I'll miss you. (我会想你的。)
I'll miss you, too. (我也会想你的。)
I'm gonna miss you. *gonna 是going to...的缩略形式，是比较随便的表达方式。

我真希望能和你在一起。
I wish I could go with you.
I wish I could go with you. (我真希望能和你在一起。)
So do I. (我也是。)

请代我向你的家人问好。
Please give my regards to your family.
Please give my regards to your family. (请代我向你的家人问好。)
I will. They asked me how you were doing. (我一定带到。我的家人也问过你的情况。)

你一定回来啊!
You must come back.

有空给我打电话。
Give me a call sometime.
Call me sometime.

我们什么时候再聚吧。
Let's get together again sometime.
Thank you. I enjoyed it very much. (谢谢，我真是太高兴了。)
Let's get together again sometime. (我们什么时候再聚吧。)

请给我写信。
Please write me (a letter).

我会给你写信的。
I'll write you (a letter).

让我们保持联系。
Let's keep in touch. *keep in touch with... “和……保持联系”、“保持接触”。
Let's keep in touch. (让我们保持联系。)
Will do. (好的。)
Let's keep in touch with each other. *each other “互相”。

别忘了写信。
Don't forget to write.
Don't forget to write. (别忘了写信。)
I won't. (忘不了。)