Write a recursive function that reverses the input string

I've been reading the book C++ For Everyone and one of the exercises said to write a function string reverse(string str) where the return value is the reverse of str.

Can somebody write some basic code and explain it to me? I've been staring at this question since yesterday and can't figure it out. The furthest I've gotten is having the function return the first letter of str (Which I still don't know how it happened)

This is as far as I got (An hour after posting this question):

string reverse(string str)
{
    string word = "";

    if (str.length() <= 1)
    {
        return str;
    }
    else
    {
        string str_copy = str;
        int n = str_copy.length() - 1;
        string last_letter = str_copy.substr(n, 1);

        str_copy = str_copy.substr(0, n);
        word += reverse(str_copy);
        return str_copy;
    }
    return word;
}

If I enter "Wolf", it returns Wol. Somebody help me out here If I return word instead of return str_copy then I get a w If I return last_letter then I get an l