他们在唱歌英语可以用"They are singing"表示。这个短语来源于英语动词"to sing"的现在进行时，表示正在进行的动作。

例句：

They are singing a beautiful song. (他们在唱一首美妙的歌。)

I can hear them singing from the next room. (我能听到他们从隔壁房间里唱歌。)

The choir is singing in harmony. (合唱团在和声中唱歌。)