I split out the modf call into it's own line so you can more clearly see what it's doing - splitting the double at the decimal point.

backtomath.c

//
//  main.c
//  Back to Math classes
//
//  Created by Curt on 2/5/13.
//  Copyright (c) 2013 Curt. All rights reserved.
//

#include <stdio.h>
#include <math.h>


int main(int argc, const char * argv[])
{
    //here's a trick - you can assign other names to *point* to x  <--notice I used the word POINT (as in, pointer)
    double x = 3.453, fractional, integer;  //so this means that x = 3.453, x= fractional AND x = integer
    int z = 125;
    double i = 2.354;
    
    
    //pull out your math into a separate line
    fractional = modf(x, &integer);  // notice the & symbol?  that's a reference to x
    
    printf("The fractional expression of %.3f is %.3f\n", x, fractional);
    
    printf("The integer expression of %.3f is %.3f\n", x, integer);
    
    printf ("The cube root of z is %f\n" , cbrt(z));
    
    printf ("The maximum value of x and i is %f\n" , fmax(x,i));

    
    return 0;
}