In some countries, it's very rude if a child talks to an adult without a subject, even when there's a word like "please" in the sentence.
For example, it's very rude if a child says to a grown-up "put the pen in the box please", because it has no subject.
The child should say "You put the pen in the box please" (literally translated from my mother tongue - Vietnamese) or "Could you put the pen in the box please?".
So, when my children say "put the pen in the box please", I will tell them "you need a subject in your sentence when talking to grown-ups".
Is it idiomatic to say "you need a subject in your sentence when talking to grown-ups", or do we have a similar expression in English?