It came to a well.In the well,it hear a frog.这里为什么可以省去frog's vioce?

问题描述:

It came to a well.In the well,it hear a frog.这里为什么可以省去frog's vioce?
如果加上frog's vioce,还需不需要去掉"a"?

很多时候不需要用 voice.
I hear you.
Everyone hear the teacher very well.
frog's voice 是很累赘的说法,也不合习惯.