...which one is correct
training the ear
training the ears?
we all have two ears so the plural one should be correct, right?
Normally you would not talk about "training the ear(s)". You would have to give me a sentence with this expression in it for me to judge.
We do say someone has a "trained ear". It is usually not plural. It means that the person is exceptional at hearing sound for some reason...perhaps music or he is consciously listening hard to distinguish a sound from other sounds
He trained his ears on the faint tapping, hoping it was a signal from his lost friend.
My friend has a trained ear for music.
how about hand?
if Im saying the following to many people
"please raise your hand."
"please raise your hands."
should hand be plural or singular. one person has one hand but if im speaking to many people. together they have more than one hand;therefore, it should be plural.
It would depend on whether you wanted the people to raise one hand or both hands. Yes, you are talking to a roomful of people but your instruction is aimed at each of them individually.