How do you know that 2-3 is in simlest form?

You know that an expression is simplified when there are no like terms left to combine. But 2 - 3 can be simplified further, it equals -1

To have a number in simplest form is to have the number at a point where it can not be divided into anymore whole numbers.  For example, 4/8 can be simplified into 1/2, but 3/5 can not be simplified.  There is nothing they can both be divided by to become whole numbers.  So, let's walk through 2-3.  2 - 3 = -1.  therefore, we can change the problem into 2/1 - 3/1.  We don't have to, but it might help.  Then we know that 2/1 - 3/1 = -1/1.  Then we look at -1/1.  Can it be divided by anything to become a new whole number?  No.  Therefore, we can safely take away the denominator, and call it -1.  So -1 is in simplest form.  :D  I hope that helped.