Answer 1

Surely will you know why if you continue learning.
In C++ (and most other programing languages), texts with double brackets are STRING CONSTANTS, and can be passed to string variables. For example:

string hello="hello";

Here [hello] is a variable name which cannot contain a bracket while ["hello"] is a string constant.
Both

cout<<"hello"<<endl;

and

cout<<hello<<endl;

are displaying the value of the corresponding symbol.