我们换个话题吧!

Let's change the subject.

Let's talk about something else.

Let's talk about something different.

I'd rather talk about something else.

言归正传。

Let's get back to the subject.

Let's get back to the point.

Let's get back on track.

我现在不想谈那件事。

I don't want to talk about it now.

I'd prefer not to talk about it.

Let's talk about it later.

We're playing golf this Sunday, right?

另外我们说说……

To change the subject...

I can't believe it.

好了，玩笑就到此……

Well, all joking aside,...

Ha, ha, that's a good one!

Well, all joking aside, let's get to work.

Well, seriously,...

顺便提一下……

By the way, how is John?

Well,...

Now,...

你刚才说什么来着?

You were saying? *说话过程中话题改变时。

Continue. Carry on.

Please go on.

啊，我想起来了。

Oh, that reminds me. I can't go. (啊，我想起来了，我去不了。)

别说了。

Let's stop talking.

I'm tired of talking.

I don't want to talk anymore.

那事儿我听过了。

I've heard the story before. *用于对方重复说一件事时。

You've already told me.

I'm sick and tired of hearing that. *sick and tired of... “对……腻透了”。

I've heard enough about it (already).*加already起强调作用。

I don't want to hear about it anymore.

这件事别再说了。

Let's drop the subject. *drop 除了“落下”之外，还表示“(问题、事件、话题等)完了、结束、停止”。

Let's drop it.

I don't want to talk about it anymore.

别那么大声说。

Don't say it so loud.

I thought you were hard of hearing.

Don't say that kind of thing so loudly.

(Please) lower you voice.

（来源：网络）