Difference between Pre and Post Incrementing a variable

Question

I have a problem understanding the nuances between i++ and ++i.
I have run several loops and changed the position of the "++". I noticed that sometimes the out put differs and sometimes the output remains the same.
Is there a rule of thumb as to whether a variable show be post or pre incremented?
Thanks

Answer 1

Thank you all.
After I asked the question, I did a little research and found this (from JGuru shortcourse):
Note that x++ and ++x are equivalent in standalone contexts where the only task is to increment a variable by one,that is, contexts where the ultimate evaluation is ignored:
int x = 4;
x++; // same effect as ++x
System.out.println("x = " + x);
This code produces the output:
x = 5;
The next part after this is:
In the following context, the placement of the increment operator is important:
int x = 4;
int y = x++;
int z = ++x;
System.out.println(
"x = " + x + " y = " + y + " z = " + z);
This code produces the output:
x = 6 y = 4 z = 6
======================================================================
I thought the out put would be x = 4, y = 6, z = 4.
I totally lost! How do I interpet this correctly?