乌鸦开始唱歌了，可刚一唱，肉就从嘴掉下去了。（用英语怎么说）

the raven begins to sing,but the time he opens his mouth,the meat falls （from his mouth.）
叙述性故事用的永远是一般现在时。

Once the raven begins to sing,the meat just falls from his mouth....
当乌鸦一开始唱歌,肉就从它嘴里掉了下来

The crow started to sing,but as it began,the meat fell off its mouth

The crow started to sing. As soon as it opened its mouth, the meat fell down.

The crow started to sing, as soon as might just sing, the meat fell down from the mouth.