Why can't a function go after Main

You can, but you have to declare it beforehand:

void myFunction(); // declaration

int main()
{
   myFunction();
}

void myFunction(){} // definition

Note that a function needs a return type. If the function does not return anything, that type must be void.

You cannot use a name/symbol which is not yet declared. That is the whole reason.

It is like this:

i = 10;  //i not yet declared

int i;

That is wrong too, exactly for the same reason. The compiler doesn't know what i is – it doesn't really care what it will be.

Just like you write this (which also makes sense to you as well as the compiler):

int i;  //declaration (and definition too!)

i = 10;  //use

you've to write this:

void myFunction(); //declaration!

int main()
{
   myFunction() //use
}

void myFunction(){}  //definition

Hope that helps.