What is wrong with this?: אתה עונה אותו
The verb "to answer", לענות, is always followed by ל, because effectively you are answering to someone.
Why do we use 'אתם' here? When do we use it and 'אתה'?
You could say "אתה עונה לו" When it says you, it could be את, אתה, אתם and אתן English does not differ in plurality or gender, and it has to be figured out by context what "you" means. In Hebrew it is not that ambiguous.
aten onot lo?