a[i]

a[0] is at address a + 0
a[1] is at address a + 1
a[i] is at address a + i

a[i] means *(a + i)
i[a] means *(i + a) = *(a + i) = a[i]


   int a[] = int[10]
double b[] = double[10]
  char c[] = char[10]

a[i] means *(a + i) where +i means +(i ints)    means +(i*4 bytes)
b[i] means *(b + i) where +i means +(i doubles) means +(i*8 bytes)
c[i] means *(c + i) where +i means +(i chars)   means +(i*1 bytes)

In general,
a[i] means *(a + i) where +i means +(i*sizeof(a[0]) bytes)