Two Questions About Two Programs

[Alienspecimen]

Program Number One:

#include <iostream>  // cin, cout, <<, >>
#include <string>    // string
using namespace std;

//The program starts below

int main()
{
    cout << "What is your first name? ";
    string firstName;
    cin >> firstName;

    cout << "\nWelcome to C++, " << firstName <<"!\n";


}

My first question is: Why do I get an error if I modify the cout statement as such:

cout << \n"Welcome to C++, " << firstName << ;

Program number two:

#include <iostream>

using namespace std;
#include <string>;


int main()
{
    cout << "Hello world!" << endl;
    return 0;
}

How come that it does not work if I modify it like this:

cout << "Hello world!" << ;

...but it works if I change it to this:

cout << "Hello world!" << "\n";

(Note the first cout statement in program number one)

What rules am I violating?

I apologize for asking such basic questions, but the book I am learning from does not explain it.

Thanks in advance.

Best

[whiteflags]

So normally you would get an error like this:

C:\Users\jk>g++ sandbox.cpp -o sandbox.exe
sandbox.cpp: In function 'int main()':
sandbox.cpp:13:50: error: expected primary-expression before ';' token
     cout << "\nWelcome to C++, " << firstName << ;
                                                  ^

When the compiler says it expects a primary expression, it's really trying to say it expects a literal constant (such as a "string", 'char' or number) or something with a name (for instance a variable). A program cannot be compiled if you have the stream insertion operator and the semi-colon with nothing in between. To put it even simpler, all you've done is end a statement right after using an operator that needs to have things on both sides. This error is no different than this perhaps more obvious problem:

int main() {
   int answer = 2 + ;
}