"Who is the boy?"
Translation:Kdo je ten hoch?
I'm having difficulties between to and ten and when to use either. I used Kdo je to hoch
Here you are asking about a particular, identifiable boy ("the" boy). Because hoch is a masculine animate noun in Czech, the demonstrative pronoun needed is ten, rather than to, which is neuter. This is not the often-used to je construction.
Chlapec is also accepted. But we cannot tell you why your answer was rejected without knowing your complete answer. Using the Report button lets us see exactly what you wrote.
I think this question lacks flexibility. I typed 'kdo je ten kluk?' and it was considered wrong. I guess 'chlapec' would have been considered wrong as well.
Your guess very wrong. All three forms kluk/hoch/chlapec are equally fine and accepted. If you have some problem, you must always report the complete sentence. Often the problem is somewhere else.
And if you believe your answer should be accepted you should use "My answer should have been accepted".