Why we add zero on console.write("hello{0}")

Answer 1

You need the braces and the number, when you have an optional argument to substitute this {0} with an Integer, character, Double, etc.

Else it is not required.

The number indicates the argument to substitute into, and the numbering starts from 0. Eg :

Integer age = 17;
String name = "Guy1";
Console.Write("{0} is {1} years old.", name, age);