Difference between Expression and Statement in C

/* diff_statements.c */
#include <stdio.h>
#define SIZE 10        /* SIZE symbolic constant */ 
 
int main(void)
{
    int index, sum;    /* declaration statement */
    int boys[SIZE];    /* array declaration */
 
    printf("user enter no of boys in 10 classes...\n");
    /* function statement */
 
    for (index = 0; index < SIZE; index++) 
        scanf("%d", &boys[index]);
 
    /* for statement with single statement */ 
 
    /* IT'S A GOOD PRACTICE TO REVIEW IF U ENTERED CORRECT VALUES */
 
    printf("No. of boys you entered for 10 classes are as:\n");
    for (index = 0; index < SIZE; index++)
        printf("%d\t", boys[index]);
    printf("\n");
 
    printf("Summing up & Displaying boys in all classes...\n");
 
    /* for statement with block of statements */     
    for (index = 0; index < SIZE; index++) {
        sum += boys[index];
        ;   /* Null Statement; this does nothing here */
    }
 
    printf("Total boys in %d classes is %d\n", SIZE, sum);
    return 0;
}

    x + y;    /* x + y is an exp but x + y; is a statement. */
    5 + 7;    /* similarly as above */
 
    while (5 < 10) {  
        /* 5 < 10 is a relational exp, not a statement. */
        /* used as a while test condition */
 
        /* other statements in while loop */
    }
 
    result = (float) (x * y) / (u + v); 
    okey = (x * y) / (u + w) * (z = a + b + c + d);
 
    /* (z = a + b + c + d) is a sub-expression in the above */
    /* expression. */

    int man; /* declaration statement */
    int man  /* removed the semicolon */
 
    /* But man isn't an expression because exp don't have types */  
    /* associated with them. */

    for (index = 0; index < SIZE; index++)
        printf("read in next value.\n");    /* for ends here */
 
    for (index = 0; index < SIZE; index++) {
        printf("read in next value\n");
        scanf("&d", &boys[index]);
    }    /* for statement with compound statement */
 
    if (celcius == farenheit)
        printf("display the temperature.");
        /* if ends with a semicolon */
 
    if (celcius == farenheit) {
        printf("fine!\n");
        printf("let's check some other temperature\n");
    }    /* if ends with closing brace of compound statement */