Interested in Language
My friend said "reply TO you" is correct, and not "I will reply you." Why????
Please explain in grammatical terms
In grammatical terms, 'reply' does not take a direct object. It's not a transitive verb. So you don't "reply a person."
In contrast, 'answer' can be transitive and take an object. I will answer the phone, I will answer the question, I will answer you after I've had a chance to think about it for a bit.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.